now this brings back memories of when I first started working professionally in darkrooms in the early 80's - which is the last time I used deep tanks!
We had a couple of DIY solutions to prevent oxidisation. The simplest one was to use a lid on the tank, as this stopped most of the problem - a print tray turned upside down worked for us..
As for a floating lid I use a sheet of "plastic foam" - sorry I can't recall the acual name of the foam, but it's white, very light and rigid - it's often used in packing of electronics products etc. It's cemically inert, and floats. You cut it with a craft knife. I seem to think we made acouple of small holes in it, threaded a piece of thin plastic rope through the holes and tied knots on the underside to give us handles..
They seemed to work very well, and if one ever needed replaceing it was only a 2 minute job to make another one..