Not sure you're size but without diapers I'm a 28 to 30 inch waist, and I also have larger thighs. I think I been through exactly what you're talking about. I was constantly changing loose leaky pull-ups for years after I should have been in diapers because of this.
Tranquility ATN's and Depends Briefs (the ones with three tapes) both fit smaller than other brands, at least in my experience. I wear a Medium in ATN's and "S/M" in Depends. I tried the Small ATN's first, but they barely fit and the bottom tapes would always snap way before the diaper needed to be changed. If your smaller than me they might fit, but Small ATN's are cloth-backed (Medium and larger are plastic-backed) so they're not ideal for IBS even with plastic pants over them (which you should definitely be using anyways).
Depends aren't great for IBS (I wear them on their own a few times a week for working out and messing in them sucks) but if you can change immediately they'll contain everything other than odor for a few minutes.
It seems like I'm always recommending these, but Medium ATN's are probably your best bet, especially if you play around with the lower tapes. You basically want them either angled "upwards" (without "crossing" the "scrunchy" part of the where the leg gathers meet the front) or straight across much lower to make a "cuff" around your thighs. The fit is really boxy so either way you should angle the top tapes down. If they're on correctly they won't rise higher in the back than the front, so pretty much just like they appear in the picture on the pack's plastic wrap.
I hope that makes sense. I've never uploaded pictures here before, but if seeing how they fit on me would help you I can try.
