I have the same tubs as you (sterilite locking with lids) and three of my snakes have the same sores on their chins. I went to the vet because one's head became very swollen and small sores started happening inside the mouth too - in this particular instance, there were two sores that lead my vet to believe a rat bit my snake twice in the mouth., in addition to chin spots. However, with the next occurence, all three of my snakes skipped eating a week before these sores showed up for the second time, first on the outside of the chin and then I checked their mouths and saw a few small sores inside that could resemble bites but no rats were present. No one seems to have any definitive answers, so I am treating it as mouth rot and administering tylan 200 which seems to be helping A LOT. As in no more swelling, shedding out and sores are gone, etc. I think at this point it may be tubs that allow snakes too much rubbing/ face squeezing areas in addition to the factors that cause stomatitis or mouth rot - lowered immune system and imperfect living conditions.