I use a flat plastic lid from a tupperware container for mine. If the edges are still too high, you can just cut the edge off on one side using scissors
Yes I've also discovered that if the Rubbermaid tote bin I have mine in gets too much moisture they will surely drown and die. I do not keep a lid on the bin any more and that has also helped out a lot!
well maybe im a bad roach mama but i just toss in some greens and orange slices, occasionally something different .. no place setting for them.... they've been thriving for years..