Take three pieces of wood from dismantled pallets. Nail/screw them together in a 'u' shape. Started screwing but it was hard work, quicker to bash a few nails in.
Screw this to the outside of the door. Had to screw because one bash with the hammer would have broken off the remaining bits which I needed to attach this to, and probably the whole door would have come crashing down, ha ha.
Then screw another slightly shorter piece to the inside of the new bottom, between the two uprights. Now what am I going to put in that new bigger hole? Bingo, found something the exact size. Amazing, no adjustments needed, a perfect fit.
Just need to cut the cat flap out with a stanley knife. An old plastic lid from a plastic box, which has since broken up and gone off in the dustcart.