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Building a Dobsonian Mount
All sizes for your Dobsonian mount depend on the size of the telescope you plan to use with the mount. Make sure your mount is sufficiently wide and heavy that it does not tip over when the telescope rests on the mount.
Use a flat board for your base. The board should be big enough to provide stability but light enough that its weight can be lifted. Drill a hole in the center of the base and insert a wheel bolt and T-nut through the hole.
For your rotating box, you can use any stable material, including particle board or plywood. Securely attach the base of the rotating box to the wheel bolt on the flat platform. Make sure the box can rotate freely. The sides of the box should be closed and of the best height for easy observations. Leave the top of the box open: this is where the telescope will rest.
Measure the diameter of the altitude bearings on your Dobsonian telescope. Cut two semicircles across from each other on the top sides of the rotating box. The diameter of the semicircles must match the diameter of your altitude bearings. Rest your telescope onto the semicircle cuts to make sure the telescope fits securely.
Advantages of a Dobsonian Mount
There are several advantages to using a Dobsonian mount instead of other, more complicated mounting systems. Dobsonian mounts are very stable, holding the telescope in place without a need for the complicated locking systems often present on tripods. Dobsonian telescopes are also very simple to move and set up. You simply place the mount on a flat, stable surface (generally a table or the ground) and fit the telescope onto the mount. This array can be disassembled quickly if you wish to move to a different observing location. The final, and often most important, advantage of the Dobsonian telescope mount is its cost. Dobsonian mounts are cheap to build and virtually free to maintain. A good Dobsonian telescope should last for years without any notable degradation in quality.
Disadvantages of a Dobsonian Mount
Dobsonian-mounted telescopes do have some limitations. You can not use any sort of sky-tracking motor with a Dobsonian mount. If you want to move the telescope, only motions by hand will work. Because of this limitation, Dobsonian telescopes are not ideal for astrophotography. If you need a telescope that can focus on one patch of sky for more than a few seconds, you should look at other kinds of telescope mounts that attach well to motors. Because of the construction of the Dobsonian mount, it is difficult to observe objects at the zenith of the sky. This would require the telescope to point directly upward, and most Dobsonian mounts do not allow for this movement.