You need to be able to hold the sponge above the water and air levels so it's in the steam. Real autoclaves either vacuum out the air or have some other method

You need to be able to hold the sponge above the water and air levels so it's in the steam. Real autoclaves either vacuum out the air or have some other method to keep the air from insulating the item being sterilized. One could construct a small wire stand to hold the sponge near the top of the pressure cooker, that would probably help.