Hi justj,
We all have gone through this dilemma and prefer different solutions.
There are several approaches to this. First and most important, are your diaper and plastic pants the right size for your anatomy? The plastic pants should fit snugly enough at the waist to provide a little bit of support to the diaper (probably not much). If your plastic pants are the correct waist size, they should not be slipping unless the heavy wet diaper is pulling them down.
Second, if you are fastening the diaper's tapes snugly, the elastic waistband usually plays almost no role as it will be almost totally stretched out. Instead, adjust the fit at the waist by taping snugly. That is critical to holding the diaper in place. If the tapes come loose, reinforce them with a small piece of duct tape.
Now for other approaches. Try wearing a snug pair of stretchable (like Spandex) underpants over your plastic pants to provide some compression to hold everything in place. This probably is the least expensive solution and I would start off with this. Better yet is wearing a pair of athletic compression pants over everything. This really holds everything in place. Another approach is to wear a onesie (bodysuit) over the plastic pants to provide enough lift to support the diaper. Last possibility is to use a pair of diaper suspenders (also called underwear suspenders). I use every one of these methods, depending on the diaper, my outer clothing, and the weather.
I just read that you ditched the plastic pants. Probably not a good idea. If you are wearing a onesie it should support both the diaper and the plastic pants.
Justj, as everything costs money, most of us experimented with the various methods slowly and over a period of time before we found what works best for us. I think you will get lists of recommendations.
Good luck and let us know how it is working for you,
--John