If you use cardboard, sneaky little trick:
1. Soak it in Elmer's wood glue (regular glue will work, but wood gets in there better from what I understand).
1a. Make sure it's completely covered with glue, and that whatever container you put it in is air tight, so the top doesn't crust over. A large piece of plastic wrap over the top, completely touching and sealing the glue works perfect for air tight.
2. Have something that is the right shape/form what the cardboard should be (I.E., a drinking glass if the finished cardboard is going to be curved).
3. Put the cardboard over the shape/form and let the cardboard dry completely on the shape/form.
4. You now have a very hard, very lightweight form that holds up well. =^_^=
Also, if you can't sew...do you have a good stapler? I know someone who made an awesome pirate/commodore's coat using staples... and a touch of fabric glue...
As a side note, I was Ester this year and will be next as well. =^_^=