I like your materials idea here. I used styrene to make a temporary mount for my RDS. It's easy to work with, light, cheap, much more durable than paper products, and you can clean it easily when it gets dirty.
One thought though -- how about cutting at the bend and adding a hinge? this would allow you to vary the bounce angle, and you could fold it away if you have occasion to use the flash straight on. I have a Demb diffuser/bouncer and have found the hinged bounce plate pretty versatile. This would also make it more compact to carry as you could fold it in half (or almost). You could also attach a velcro strap (hooks on one side and fluff on the other) and it would be almost instantly attachable to any flash. Okay. . . that's two things. . .