Well, if you don't mind buying a bunch then there's a design you might be interested in. The main parts are a Clippard MAT-2, MJV-2, and MPA-3. The Queen's Own website has pretty good documentation about that type of cannon:
http://www.queensown.org/armament/gun-construction.html
If you have a lathe and drill press, they have great instructions for making your own breeches. If you don't, there's an alternative. OSH sells a brass T fitting that makes a perfect breech. It is in their plumbing section, with 1/8" NPT threads on the bottom and 1/4" compression threads on the side and top. These 1/4" compression threads come in handy because it just so happens that 1/4" diameter brake line (available from various automotive stores) is just the right size for BB cannon barrels. Overall, it'll cost about $30 per barrel to use them. They're great cannons, I've used them myself.
By the way, why are you aiming for only 100 feet per second? In most clubs we shoot for around 160 feet per second, with the larger-caliber shot. smaller balls go faster. Get some gun-testing foam. That's 2 inch thick white insulation foam. If shots go in one side and out the other, then turn down firing pressure. Did you get a regulator?