Yikes you got a big hole there!
You opening looks different from mine. It looks taller and narrower.
Anyway, on removing the old cover, all it takes is popping the little metal hinge wires out of their slots and the door comes off.
When I put the door on the new heater, I added a screw across the metal tab to keep the pin from falling out while under way.
Since you will not have a water heater there for some time, you will have to fashion a different way to hold the cover in place.
Let me know if you need some specific pictures or anything else.