That's easily fixable, but you will need a heat gun...

A good place to purchase them is at a local hardware store.

They are frequently sold in the electronics or tools sections.

Anyway, take the STAB and place it on a flat surface.

Heat both sides with the heat gun and then press the stab down against the surface to straighten it out.

Clamp it in this position and apply heat to remove any remaining wrinkles let it cool fully while it is straight.

If it is still a bit bowed, reheat... remember that the heat will pull the covering taught so you want to adjust your attack accordingly...

You would thing that all you need to do is to heat the convex side, but you really need to heat both sides, to loosen one ( to let it streatch ) and to tighten the other ( to pull it taught )... thereby fixing the problem.