Do you re-use petri-dishes? I hate the idea of buying sterilised plastic dishes and using them just once. I want to fill (and ideally re-fill) the dishes and then sterilise them. I found PP5 sauce containers (about 50ml zsize) useful for that. I think the higher wall to bottom surface ratio (taller shape factor) allow for some bubbling while it's in the pressure cooker, while standard petri-dishes probably boil over, stick together, and end up with continuous agar-bridges to the outside for contamination to get in.