what you can do is use card stock, it's good enough to hold together, then just paint it with resin so it'll absorb the resin and harden. Follow the resin coat with a Rondo coat.
I use standard tape to hold my stuff together, gluing takes too long.
As for Rondo, it is just resin (with out the hardener) mixed with Bondo to a nice honey thickness, then add the bondo hardener not resin hardener, mix good and poor it on.
A'den know's it a little better, maybe he'll give a better idea.
Or like Nau'ur Baatir says, some people use plastic drinking cups, I have a few from TacoHell that I plan on cutting in half just for a base and then something flat to layer on top of it.