I'm fairly new to farming mealworms, but I have had some success in creating my first breeding stock (I'm aiming for 500 beetles). Like you, I've also used a plastic bin with rolled organic oats as substrate (mixed with bran and wholegrain wheat and rye flours). So far, I have had very few casualties (out of 400 pupae, only 3 have been cannibalised). In my bin, the moisture sources are evenly spread out and I have bits of chemical-free cardboard for them to climb on.
I even keep a big piece of carrot in the pupa container, so the freshly hatched beetles don't cannibalise their kin before I can put them in the breeding container. Mealworms will cannibalize each other if the space they're kept in is too crowded or they don't have enough easily accessible food and moisture. They will also cannibalize the pupa for moisture (as will the adult beetles).