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the Weber VST site - somewhere on there, there's a procedure for it. I followed it with great success to patch a couple of Celestions I got cheap off eBay.
Basically, you push the pieces of the tear back into the hole, then brush on some white glue, and lay in some toilet paper (yes, toilet paper). Then more glue, more TP. Several layers on each side, then let the whole lamination dry... good as new.
You can just barely see the patch on this G12H-80, at the top, around 11 o'clock.