I fix any leak in plastic with one of two methods, both of which supply the same end result.
1. Torch and heated knife
2. Soldering iron.

Sometimes a little extra plastic is needed for your welding bead, try to find the same plastic for your welding. So far it has worked on every leak I have had to seal.
It works awesome on cracked solar panels and you do not lose that particular vein in your panel, so in turn, the panel does not lose efficiency.