In fact, only two are firmly screwed into the rear deck; the third is on the edge of the hole, and thus isn't firmly tightened down. The fourth just floats free, so I removed it.

You might consider making an adapter ring out of plywood and sealing it to the speaker hole with RTV, then screwing the speaker to the adapter.

For rear deck speakers, your bass response is often improved by sealing around the edge of the speaker so that the cabin and the trunk are two different (sealed) air spaces. In this situation, the trunk becomes the speaker enclosure.