I use the Ball brand plastic storage lids a have used them over and over in my pressure cooker with no problems. I got sick and tire of the rust too, plus with syn. filter discs, you can drill a 1/2" hole to reduce evaporation while allowing spawn to grow. I use 1/2 gal wide mouths now, but i used to use 1/2 pts too. 1/2 gals are so much mor efficient, if your pressure cooker is large enough to hold them. For the 1/2" hole, I use a spade bit and have not had one lid crack, for the 1/2 pt type I use a 3/64 drill bit.
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-ChesterCopperpott-
"Old enough to know better, young enough to do it anyway."