Patches says "it needs some bolts right through each frame and the door".
I installed a cat flap in the side door of Ari's garage this morning. It's a double steel skinned door, so there were loads of sparks. I wasn't very impressed with the flap... It was only designed fixed with four tiny screws on one side and double sided sticky tape on the other. Presumably because you can't really expect average Joe Soap to drill 4 holes in a door that all line up with eight holes on the frames. So after a trip to Home Depot I drilled out the frames and door to take four 3 inch bolts and trimmed off the ends with Ari's Dremel to make a neat job.
I love Dremels... One of the best tools ever.
Job done and I've just awarded myself a bottle of Taddy Porter.