You should check out packrafting forums for info on home made inflatables. The community is very active, and while there aren't a ton of home made projects there are a few. This forum at BackPackingLight (http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=20903) has some good information on materials to use.

I also found a few write ups which are pretty detailed and easy to understand (even after Google translated two of them from German). This one is a home made PVC packraft (http://www.packrafting.de/2011/12/myob-make-your-own-boat.html), and the maker explains what adhesives and seams to use and the placement and production of tiedowns (http://www.packrafting.de/2011/09/tiedowns-richtig-ankleben.html)

It also seems that people have had success contacting people at Stephenson's Warmlite (http://warmlite.com/) for advice given that they make simple but rugged inflatable mattresses, or even talking to the folks at Alpacka (http://www.alpackaraft.com/) who are the current kings of super tough packrafts.

Here are a few more: Packrafting.org (http://packrafting.org/forums/) Backpacking Light's Packraft forumhttp://www.backpackinglight.com/cgi-bin/backpackinglight/forums/display_forum.html?forum=56 Flyweight Designs (newcomers to the ultralight packraft world, but very successful) http://flyweightdesigns.com/default.aspx