There is a post somewhere on the forum about a guy that used a layer of thick play-doe like sealer that would stick the BBs in. He just hang his target in front of it.
I don't know the english therm for the substance used, but it was flexible, moldable and sticky to some extent. Like the stuff you use to seal around windows sometime.
The thread is recent, like 1 or 2 week old.
Edit: here you go:
http://www.airsoftcanada.com/showthread.php?t=119181