Installing a Bathtub that does not have a flange
I hate to give an answer that's not really an answer but I think you need to return that tub. Without a flange there's no way to guarantee that water won't migrate over and around the edge of the tub into your wall where cavity where it will fester, unseen until you have a major problem. If someone has a better, more can-do, solution I'll be the first in line to delete this answer but until then, sometimes you just have to cut your losses:(