Floating lid for large trays?

Use another tray as a lid!

I just put another tray in on top of the developer. It floats as you'd expect...

I use the old standard Paterson trays that nest together when stored dry as most matching trays do. Popping an extra one bottom-down on top of a same-size tray that contains developer covers the surface almost completely and buys some time if you're doing close-together printing sessions and would rather not clean up in between.

You don't have to get your fingers wet when placing nor removing because of the rim that sticks above the other rim. Just have some wipes handy to wipe the bottom as you lift it off if you're not doing it in the sink (my case).

And don't forget that those 2 dry looking trays sitting there actually could cause quite a mess if you put someting in the top one or go to lift them together.

Cheers.

I use a closed cell foam called Plastazote LD33. It's nitrogen blown and won't react with the developer.

I use pieces of plastic cut from white kitchen trash bags and float that on the trays. Works a charm, is inexpensive and can be replaced new for each use.

As noted by Mike Te, I use a second tray, most trays will nest. I normally use a tray as a lid if I'm taking a break, otherwise I toss the developer and return the stop and fix to bottles.

I hope the OP hasn't had a floating tray lid in place for 14 years since this thread started!
That being said, this sort of question is an example of one where it makes sense to add new input to old questions.

If it is to be any significant length of time, pour it into a glass bottle, top with fresh solution, and cap.