A lot of things come to mind try the local lumberyard for sheets of vinyl or formica like countertop material.

Those materials don't seem any ligher to me here is what I have thought of.

Skin on frame kayaks use 12oz per yd polyester or nylon fabric that would be the lightest.

I would lay up some fiberglass or carbon fiber cloth between sheets of waxed glass to produce some stiff but flexable epoxyglass sheets. One layer of 4 oz cloth would be about .005 thick. I used that method to make 5" tubes for model jet ducting it is smooth on both sides that way and ready to go transparent or painted.

Substitute the plywood deck wood for thinner plywood 1/16th is pretty common in birch and veneers go down around 1/32.

I read on another thread about the shells they offer a light comp version but haven't found it yet.

probably the lightest material is aircraft Darcon.  There is a 1.8oz option that is heat shrink (you can use a standard household iron).  Then simply paint it with multiple coats to get a water tight finish.  I epoxied mine with three coats (two coats she was still leaking) and it is water tight.  I'm planning 2-3 finish coats of Sikkens for colour and UV.  The stuff is made by Poly Fiber and you will want to use their glue (Poly-tak).  I used the heavy duty 3.4 oz option for my hull and deck.  Two late season paddles last year and she did fine.  Time will tell on the longer term.   I have been flying with this stuff for 25+ years and it is strong. 

http://www.aircraftspruce.com/pdf/2009Individual/Cat09CS.pdf

http://www.aircraftspruce.com/catalog/cmpages/dacronfabric.php

 Also take a look at the airolite boats website for more info.  http://gaboats.com/ - done right the stuff is tight as a drum and water tight... and of course super light.