Are you aware of a product call " Lexan ", we use this product in the hobby
business, specially for the RC Car body's. It's a very thin, transparent,
strong & flexible plastic.

You can cut it with a scissor, make some small strip and glue those one
with "Cyanoacrilate" glue, or more commonly known as "ZAP" glue (Also use
in the hobby business) to reinforce the crack. Both can be bought at any
good hobby shop.

Mine was repair like that, it looks a bit like a "zipper" but it's not
leaking.
The lower part of the skylight (original one as 2 thickness) was cut /
remove (to much damage) with a Dremel tool and I kept the upper one (good
condition)

It's been one year now and still looking good.

Ben
http://pages.infinit.net/eurokit/