there's not a lot you can do to keep a diaper fitting well if you have a pot belly. Just remember to fit the top tapes along your belt line, wherever that happens to end up being. Then tape the lowers so they seal around your legs. Don't try attaching the top tapes low on the front of the diaper. The diaper may bunch up and fit a bit weird between the lower and upper tapes, but that's just how it's going to have to be until you lose some weight
I use control top pantyhose to keep my diaper from sagging when wet, but also does a great job of keeping it snug and in place from top to bottom. If you don't want to wear the whole thing the answer is to simply cut off the legs just below the "panty" and presto, the perfect diaper support. I normally use Leggs Sheer Energy, size Q+ (I'm 5'10", 220 lbs.) but almost any pantyhose that fits should work. I know this may seem a bit unusual but hey, you're already wearing a diaper, so just one more unusual undergarment really isn't that big of a deal.