IN the link I gave above called 'nestable pallets' you can see that the 'feet' of the pallet go down into a socket and they nest flush to each other when the foot is in the bottom of the socket. I would say you could put a piece of wood or some other material that doesn't break down in water that is the desired width of the gap you want between pallets. You could probably only put the 'spacer' in the holes on the 4 corners of each pallet. The weight of the pallet above would keep it in place. This would I think keep the gap permanent and you wouldn't have to worry about rocks rolling out of place.

I agree you could weight the pallet condo down with some rip rap, limestone or other jagged edged rock material and create crevices or crayfish hiding spots too.

I'm not aware that FHM care so much about the gap being too wide. When my FHM spawned I never saw my pallet condos being used and instead I think they went inside a section of smooth walled plastic culvert and did the deed there.

There is a youtube video link somewhere that shows FHM spawning, you can see by all the frantic activity of the male that they probably don't mind a little room to swim around with all the wriggling going on.