[UPDATE: you find full Instructable here]
Times ago I've built a panohead with wood profiles to shoot spherical panoramas (see www.andreabiffi.com) with my Canon EOS40D and Tamron zoom 11-18mm. You can make a similar one (easily a better one) with a few suggestions.
Construction is very simple, the tricky part is to make a strong connection on the vertical joint. Indeed you need to have a little knob to tighten that, and in the same way you need the joint to be firm and strong enough to keep the weight of the Canon 1D camera and the lens. I've found a solution gluing to vertical brace and to moving complex, two plates of rough plastic, so that they're compressed one to each other by the knob. After degrease the surfaces, you'll see that a little strain on the knob will lock the parts together in a great way.
Some times you could shoot from the slope of a mountain, or on a sharply-sloping road.
In these situations adjusting the horizon in postprocessing is very difficult, also impossible in some cases (you should have a vertical line in the panorama). So I really suggest to take the first photo exactly horizontal, and the two levels on the panohead help you to do that very well.
To set the exact lenghts of the braces and the distances for camera anchor bolt you need to follow some tutorial you could found with a "nodal point" research on google. Every lens has his own nodal point, which has to remain absolutley fixed both on horizontal and vertical rotation of the panohead.
A last note about the compass. Apart from the fact that it's very cool ;-) it gives you an approximate angle between one shot and the other. Beware that compasses are influenced by iron proximity, this is the reason because near plate is made of aluminium.
This panohead need a rotating head on the tripod, but most of times this is not a problem. There are a lot of DIY solutions for this equipment, some grosser of mine and some more accurate. Of course a lot depends from you skills and your tools, but in my opinion to choose one project or another you have to focus on three points: cost, weight and precision. My equipment is with no doubt cheap an light, and it's very good to shot panorama to be developed with specific software. But I could never shoot a gigapan with telephoto, because I've no precise steps in rotation, and also the structure is no rigid enough (you need aluminium for that).
Write me for every question. Bye