I found that for some quick and dirty modeling, clear plastic did work, good tip! It was more of a proof of concept rather than an actual design, but I might try a full scale mock up with it.
I know with all the packraft kits the TPU is on the outside, but my design would leave the TPU on the inside, basically attaching two flat sheets together. Is that going to be a problem in terms of durability or anything? I was hoping to use the single sided 7 oz fabric.