The lack of a fuse box cover interests me as mine came with an obviously home made cover. A box of glued together plexiglass bits. I don't remember how it was held in place when I got the car but, now it is held by a strip of Gorilla Tape. I just upgraded the top fuse holder that supplies the fuel pump with a small brass wood screw. The fuse seems more secure now and I expect won't give trouble again, until it gives trouble again. I've concluded that one does not fix these cars, one just fiddles with things until things work until one must fiddle with things again. But, I wouldn't have it any other way.
My Spitfire was the same, no fuse box cover, and in the much damper UK, it caused a bunch of issues. My temporary/permanent fix was to put cling wrap over the box body and hold it in place with rubber bands...