I will often sterilize petri dishes before using them in a pressure cooker. The safest way I have found to do that is in a mason jar. However, I have to fill the mason jar upside down, which complicates getting them out. So I designed this mason jar insert to hold the petri dishes, making it easier to extract them after sterilization.

If you plan to use this inside a pressure cooker, you MUST use a plastic that is heat-resistant up to 121 °C (250 °F). I have had great success with High Temperature Polyamide (PAHT) that is carbon fiber reinforced. After 2 hours in the pressure cooker at 15psi, these come out exactly how they went in.

You may be able to get away with using non-CF reinforced PAHT (which is heat resistant up to 120 °C), but I haven't bothered trying this specific filament. Acrylonitrile Butadiene Styrene (ABS) and Polypropylene (PP) are both heat resistant up to 100 °C and may be useful in some circumstances, but should not be used in a pressure cooker. Other plastics, such as PLA and PETG, are fine if you just want to have an easy way to store petri dishes in a mason jar.