I had been wanting to make a Captain America shield for some time, but had a hard time finding a suitable item to make it from. I originally tried using an old plastic sled, but it gave me all sorts of grief. The type of plastic would not take paint very well (not even Krylon Fusion for plastics), or any type of adhesive that I tried.
I eventually came across a used satellite dish, and with a little bit of modifying this turned out to the be best option for a Captain America shield.
If you are interested in making one of these, you may have to do a bit of searching and asking to find a used satellite dish. I've seen them show up at thrift stores from time to time. Keep your eyes open, and you should be able to find one.
If you don't have access to an old satellite dish I would highly recommend using a round metal sled, like this one from amazon.
I have a separate instructable on how to make a flyable version out of cardboard and duct tape, which you can check out here: Flying Captain America Shield.
Thanks for looking!