You could try to cover the plastic grille with a piece of landscaping fabric which is a kind of synthetic fabric that lets water through. It won't rot, so you don't need to replace it. Also sometimes things come in post packed in a white woven plastic bag, this can also be used, although the cut edges will fray and you may end up with bits of plastic in the compost. If you cut the piece so that it comes up the sides, you can also use it to remove the compost easily.
Andrew, try Wiggly Wigglers, Worms Direct UK or VermiSell for more worms by post.
I'm not Bing, I'm his successor at CRISP but can't find out how to unsubsrice and resubscribe as myself!