Hi Max;
I see many ways to utilize that locker as a bell.
I can only assume that you want to keep the original look on the front.
The louvers would need to be covered on the inside, possibly by welding plates over or as simple as tin foil and metal duct tape.
Sealing the door depends on how it is built.
If the door sits against a flat plate, than
superwool gasket would seal it up.
If the door has no "seat" to
land on then stuffing with superwool or nicely welding the crack shut would do the trick.
For a cleanout.
Myself, I would try to keep the door hinged and have access to the entire bell .
You could dry stack brick directly inside the door as part of the mass.
Easy to remove when accessing the bell.
Or you could cut a hole in an end/side and make a larger removable "patch" .to cover it.