[Edited to note:] Here's a kiwi page -- as it happens, Gary Dierking's, author of the afore-cited book -- on building amas from blue foam board and 'glass: http://homepages.paradise.net.nz/garyd/quikama.html. He says this about the design presented:
Other than a solid log, the foam and fiberglass ama is in my opinion the easiest to build. I'm using the method employed to build tens of thousands of surfboards since the 1960's. The ama design used here as an example would suit a shunting or tacking canoe of 16'-18' (4.8-5.5M) It can be connected to the cross beams by either a direct (Hawaiian) connection or an indirect (almost everyone else) method using struts.
Chesapeake Light Craft sells plans (and kits!) for a stitch-and-tape kayak/canoe sailing kit. Verra shapely, too:
Could do worse than to build that (or buy the plans and use them as a starting point for your own design.