Apparently the different brands of bag all use different seals to accommodate different skin types, there's also wafer which are supposed to help with the seal.
I'm fairly normal in most things but I couldn't find a stoma bag that would stick to my skin reliably, even using the wafers, fortunately the temp ileo was temp (3 months only).
I once got through 8 bags in a day, although when I did finally get one to stick, of it lasted 24 hours it would last a few days.. I was very reluctant to change them though. My stoma would not stop producing, even using the marshmallow trick, so took a lot of patience and quick hands. Apparently permanent stoma are much easier, but your not alone struggling with a temp.. try the wafers, maybe some sample bags, although by the time they arrive your op will be due.
Oh and please consider donate your left over stoma kit to 3rd world charity... I can't imagine how bad it would be living in intense heat in a wooden shack, with limited water and a used plastic bag for a stoma bag