Get some 2024T3 aluminum, 0.32 thick or maybe 0.040. Make a piece that will fill in the extra space around your new water heater and rivet the new aluminum in place. Olympic rivets, sealed with Vulkem or something similar at installation should work.
Aircraft Spruce and Specialty Co. is a good source for aluminum, or the light aircraft maintenance people at your local airport might have a source, or even some scrap they would sell you.
Clecos are wonderful for holding a new piece in proper alignment until it can be riveted, but for a one-time project, I have seen very small sheet metal screws with nuts used as substitutes. Slow, but they work.
Check for a local chapter of the Experimental Aircraft Association (eaa.org). Those guys are usually very familiar with sheet metal work and would be a good source of information.