I adjusted the thumbscrews to put a bit of a pull-in tension on the end pieces, so no, they are solid. I can slide them forward/back to clean, or just pull out the glass lids.
I was considering using double-sided tape if they had problems with sliding off. With the lids, I can't really hook the inside edge of the tank without being in the way of the glass.
Probably will replace the lids at some point, but for now, I want to keep down the evaporation.