And alternatively we could fasten locking clips like the ones shown below on the edges of the speaker plate. These would be pushed onto the edges of the grille bars (the outer edges of the outer bars, that is). That would keep the top part of the speaker plate from moving. Actually there are things about this approach (particularly from a DIY standpoint) I like the best. I need to get some and see what sort of thickness they'll work best with.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end