Building a foam dog platform/seat for dog

I have a sit on top tandem kayak and I want to have something to make the front bottom section of the kayak flat for my dog. I was thinking about using expanding spray foam to create some sort of removable platform that levels out the bottom of the kayak.

I was thinking I could use window plastic or something on the bottom to release the foam. Then spray the foam in the front section. Maybe press some type of vinyl or waterproof material over the foam to make it bit more durable.

See image attached
https://drive.google.com/file/d/0B9qKkA4yOwu3MXZWR0NkTUVpNFE/view?usp=sharing
So my questions are...

do you think this would work?
what materials would be best for the top?

any other alternative ideas?

If I was to make something like your describing. I would get some two part urethane foam (4lb), I'm sure if you search on Amazon you will find it, ( the stuff is not cheap). You will likely need some urethane mold release spray as well. Spray down the interior with the release spray then mix up some foam and pour it in, a little goes a long way, doing it in several pourings works best otherwise it can get out of control, (found that one out the hard way lol). Once you get it built up slightly higher than your final height you just take a sureform body file, disk sander, or anything that removes the excess foam quickly intil you get a nice flat surface on top. Next you just glue outdoor carpet to the top and your done. The foam won't stick to the plastic boat even if you miss a couple spots with the release spray. Shouldn't weigh more than a couple lbs, and should conform to the contour of the seating area so you shouldn't need any attachment to the boat. Easy as pie.
FE

I used 2 part expanding foam to make a seat insert for my outback and it did stick to the kayak like 'poo to a blanket' where I missed release agent, particularly as the area is textured. What I would do next time is line the mould area with plastic (eg garbage bag) and trim off the excess when set and no need for a release agent.

that sounds perfect. Is this the stuff your talking about? Smooth-On Flex Foam-It!

Yea that will work too, if it was me I would likely use both, just to be sure. BTW that urethane foam sticks like poo to your garage floor as well, I still have spots on my garage flooor from my last mis-adventures in foam (lol). It takes about a week to wear off your hands and arms as well.
FE

or maybe this stuff: TotalBoat Liquid Urethane Foam Kit 6 Lb Density, Closed Cell for Flotation & Reinforcement

and your right this is not cheap. I started out thinking I was going to buy a $10 can of spray foam or 2. Now it's looking like this is going to be a little over $100?

I don't know what stuff I used as I got it from a bath repair man that used it to fill gaps between old bath tubs and a new liner, but it worked well and stuck like the proverbial.