The inner diameter of the secondary cage should be large enough such that the light cone will not be cropped. As a rule of thumb, I choose a diameter according to that of the primary plus 7 cm. This will give you even some buffer for mounting components on the secondary cage such as filter slides, focuser baffles, etc. Alternatively, you can have the minimum front aperture calculated by the program Newt.

For bending square aluminum tubes, I make wooden templates of hard wood (such as oak or the hard wood ply wood shown on the right). The usual birch plywood is not as suited as it is relatively soft and deforms under the high loads. The radius of the templates should be somewhat smaller than the desired radius of the final rings. As a rule of thumb, you can start with a radius that is 10% smaller. The outcome may vary, however, depending on the aluminum tubing and the hardness of the templates (so it ends up with try-and-error). I cut the templates with the jig saw (take care for a vertical cut) or with the router. To achieve good handling of the templates, I attach a small piece of crap plywood at the bottom of one of the templates and three small pieces on the top (see right picture).

Take care that the tubing does not get distorted.

Generally, this method can be used to bend aluminum square tubing up to 15x15x1.5 mm or 10x20x1.5 mm. You should achieve the complete bending in a single round, as you cannot do the bending in small steps (e.g. for subsequent improvement). To achieve a lasting plastic deformation of the aluminum material, a minimum deformation is required, that cannot be compensated by the material's elasticity.

Do not heat the tubes for bending, as this will result in uneven curvature! You may fill the tubing with sand to avoid inward bending of the sides of the tubing, but I don't do this.

The spider is made to connect excentrically to the secondary holder. Additionally the spider legs are crossed, which increases the effective base length of the involved triangles. I use four wires for a spider, which are threaded through at the secondary holder.

For wiring, I use 0.8 mm spring steel wires (www.conrad.de).

The bolts are flattened with a file and then a 1.5 mm hole is drilled at one end.

At a later point, the wires are passed through these holes, bent and fixed in the vise. With pliers, the bolts are turned and the wires  are twirled. Try to to do this a few times on scrap wire.

After this step, the wire is taken out of the cage (number the wires!) and connected to the bolts (see above).

It is very important to cross the wires, as this increases the effective length of the base of the involved triangles. The crossing points are fixed by thin wires and soldering, which again increases the overall strength of the spider.

A properly made wire spider finally has a stability clearly beyond that of a conventional spider of same thickness/obstruction/weight (pick whatever is important to you).

A simple and nevertheless very robust alternative is soldering the brass sheets into slotted brass pins. This can be done even with regular solder and a conventional tip.

This yields a very robust connection of the spider to the upper ring (virtually unbreakable, even in a vice ).

The purpose of the secondary holder is to keep the secondary rigidly at a defined position in a defined orientation. Secondaries dangling on a long threaded rod of whatever size make my toe nails coil up (as we say in Germany ). A good secondary holder is rigid with the shortest possible lever arms.

My favorite type of secondary holder is a simplified version of the holder made by Stathis Kafalis for his Kyklopas (the "mother" of all 24-inch Dobs ). The holder consists of two parts made of two short aluminum tubes. The inner tube is the actual holder of the secondary.

The secondary mirror is then glued with silicone (~ 1mm thickness) onto the bottom plate. The silicone bonding is safe.

Alternatively, the spider arms can be as well mounted between two aluminum plates, as shown further above. That type is a bit easier to make.

The heating has a power of 0.5 W for my 14-inch and 1 W for my 22-inch Dob. Using a voltage of 3 V, this corresponds to three resistors of 6 and 3 Ohms, respectively (power = voltage squared divided by total resistance)

These baffles are attached to the rings of the secondary cage by patent fasteners.

More about baffling is on the page of my 22" Lowrider here.

If you observe under stray light or even with street lights nearby, you should be more generous with the size of the baffles, as light falling onto the barrel or barlow element of eyepieces will cause stray light as well.

All in all, the baffling of the entire Dob should make sure that while looking through the focuser (even at an angle), only mirrors and black surfaces are visible. You will notice that also the focuser board needs to have a black back and should have a certain minimum size.

One which side should one mount the focuser? Left or right? And what means "left" and "right" on a telescope? 

Let us define as "left" that side, which faces East, when the Dob points toward South, and "right" the side, which then faces West. My own Dobs have the focuser on the "right" side, while most commercial Dobs have it on the "left" side.

There had been several discussion on the internet groups whether people prefer to "push" or to "pull" while tracking the stars with their Dobs. Whether you use push or pull mode depends solely on the side where the focuser is mounted. As philosophical such discussions may be, they do not come to the point. 

The point is rather: Which eye do you prefer for observing?

Somebody, who observes preferentially with his left eye, is better up with the finder and the focuser on the right side of the telescope. On the other hand, an observer who prefers his right eye, has a better time while being on the left side of the telescope. Try it out!

I observe solely with my left eye. Hence all my telescopes have the focuser and finders on the right side. It's as simple as that

Focuser on the RIGHT side (my 22"):

Focuser on the LEFT side  (GSO 12")