If the problem is that you are short of money, join your local Freegle (UK based, volunteers) and Freecycle (US privately owned) and ask/look for a used door.
Misguided people are always chucking out good wooden doors and replacing it with plastic rubbish.
You will need at least a roof rack and a helper to collect.
If you end up having to pay, search on ebay within x miles of your home. My shed door cost me £1, but some second-hand doors go for as much as ten.
Be choosey and get a good one, in the correct size that you can fit with no more than a bit of planing. Test for square and flat.
Or, for a quick and fairly easy job, take out the catflap and put in a well-fitting piece of WBP ply, painted black. It will not show much. Cheapest way might be to get an 18mm offcut and use two pieces to make up the approximate thickness.
You can do a new weather bar at the same time. Paint it well, before fitting. Drill and screw from the inside of the door so there are no holes or screwheads on the outside. It looks like your door is hinged to open outwards, which is not normal.