The floor mentioned above works really well.
It's made of hardwood 9mm ply. I didn't bother with marine ply, since it's very expensive for an experiment and to be honest, if you don't treat/waterproof the edge of the plys, it's going to fall apart anyway!
Disclaimer: Follow at your own risk; eyes, fingers etc. are useful and difficult to replace. Always check yourself before you wreck yourself!
I purchased an 8x4 board (select for best quality) and had the store (Bunnings) cut a piece 480mm wide along the long edge (check the width required for your craft). They offered a free service of 5 cuts. This meant that I didn't need my table saw/panel saw, and I could get both pieces in the car
I spent a long time with the kayak deflated and the floor exposed, deciding upon the position and size of the floor. The original sketches called for fancy curves (see the image for dimensions used)
Remember: measure twice, cut once!
Once marked, I covered the pencil lines with Sellotape and burnished with my thumbnail to ensure a good hold. The tape helps to prevent break-out.
All cutting was done with a home-made frame saw and a very cheap fine Japanese kataba style draw saw. Let the saw do the work! This type of tool makes cutting straight and square very easy, even for a novice. You can of course use any cutting device you like.
After cutting, I ran a plane down to "break" all the edges (glass paper with a backing block will do) This reduces the chances that the wood will splinter.
A quick sanding with fine glass paper was followed by de-dusting with a slightly damp cloth
Yacht varnish was carefully applied to all edges, taking care to fill any voids (these should be filled really!) Once touch dry, the whole board was varnished on both sides. Apply as many coats as you like following instructions.
Edging is made from some cheap garden hose, it's purpose is to protect the fabric of the craft from chafing. I did this as follows...
Cut a length long enough to cover both sides.
Lay the hose out to remove any kinks and twists. We do not want a spiral cut!
Using a craft knife, blade-up I pulled it toward myself, keeping the tube lined up. BTW a little wetness in the pipe helps lubricate the cut. Cut a short distance, check alignment and cut again. There are probably extruded edgings available that could be used.
Take a cloth and dry the tube
I used superglue to fix to the board about every 150/200mm paying attention to the ends. I have allowed a little overlap to cover the corners, but have not yet decided how/if I need to finish the ends.
Note that the finger holes for carrying have not yet been implemented (I created them in the graphics package). I intend to add several, along with some holes in the floor to lessen weight and allow areas for attaching bindings. Note also that the image is somewhat distorted by the camera, I should alter that in the graphics!
Fitting: Very simple, lay out the kayak and pull the sponsons to the side, fit the board where required and inflate!
Note that the overall size is 480x1930

Larger image
here