I've got a rather large belly and have tried darn near every one I can find available in Canada and they all seem to have the same effect, they all roll down my stomach and start to hurt.
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
Only trick I found that somehow work is to use a onesie or plastic pants with a snug fit, they will keep the diaper from getting lower and lower down there...