warped door in C172

The left door of my Cessna 172 is warped. I've tried the benign
options of using wooden blocks at appropriate places and applying
pressure to bend the door. But that didn't get the door back to where
it has a tight seal at the lower side (opposite hinges). There's still
about 1/2 inch gap between the door and the frame. Any ideas of what
else I can try?
Dale