The original elevons that came with my kit where not of very good quality.
After two months flying they looked like this:

So I lamintaed two 2mm sheets of balsa together with 5 minute epoxy, and cut two elevons out of that.
The best stuff to stiffen these (I think) is carbon fibre tape. I didn't have any, so I used regular fiber tape. I taped pieces of fiber tape with the fiber accross for extra strength.

With these elevons I have flown, and crashed into anything I could find, and then some things I couldn't find but couldn't avoid either. They are still in perfect condition.

One other thing I changed later on, are the hinges. The instructions tell you to use one long piece of tape the entire length of the elevon. This is fine on small elevons like on my Mini Weasel, but on longer ones like the Unicorn they allow too much flutter. I found this method on the www.yourzagi.com site.

It takes some time and fiddling around with tape and greaseproof paper, but well worth the trouble! I now know what they mean when in a model review the reviewer mentions "crisp control".