Would Tyvek be a good candidate for covering the aerocap? Tyvek now comes in sprayable form if you'd want to preserve the shape of your cap as best as you can, but fabric sheets of Tyvek would be awesome as a non-rigid base layer and are pretty easy to obtain. I'm thinking of using some shreds of Tyvek (obtained from dumpsters) and the liquid stuff as a sort of heavy-duty papier-mache.
Here's a
link, if you're interested.