I regret that I am only now responding and perhaps you have already found the solution to your problem. I had bags for three and a half years and the original stoma became progressively herniated and prolapsed, like you I had a very large amount of bowel in the bag and the bag could only be attached by being prostrate. My interim solution was to wear an "elasticized back support" belt, perhaps three inches wide . I wore this if leaving home. Eventually I had just so much bowel outside the stoma and I had the original stoma closed off and an Ilestomy stoma and bag substituted.