It is certainly annoying when the BBC Micro ashtray cover is removed/compromised. It is just a thin bit of material covering a void and when gone you see the speaker and circuit board underneath. Nasty. Well, I have one of those and need to plug the hole.
Step in an old floppy disc, some strong duct tape and a pair of scissors.
Using floppy disc material, black in this case to keep it in line with the styling, has a texture similar to the BBC keyboard cover. A nice black mottled finished. I simply measured the size of the hole and cut the floppy to size. Then placing it down, stuck some tape to it and vola! It is a bodge in the true sense, but actually looks quite good.
I know it is not flush with the rest of the case, but you could bulk it out with some sticky pads, perhaps.
Anyway, for a quick fix, it looks rather nice.