Boilerman,
You and I both use the same diapers, so I am well aware of their sag factor when saturated.
IMHO, there is no perfect solution. Most solutions involve wearing something over the diaper to provide support and resist the sag.
I like wearing a onesie, such as Patrick recommended. Another option is a pair of breathable PUL pants (plastic pants). Note that these are not vinyl pants. Yet another is a pair of stretchy underpants. A similar, but better option, with more breathability is wearing a pair of mesh fixing pants over the diaper. As they are made of mesh, there is very, very little heat build-up.
The only solution not involving an extra layer may be diaper suspenders. They are worn under your undershirt or T-shirt and attach either to the diaper or to your plastic pants. The down side is that they make it more complicated when the time comes to change the diaper.
A completelydifferent approach might be to assume that you will need to wear a onsie, PUL pants, etc. on top of your diaper. If you have to wear that layer outer layer for support, why not change your diaper to a quality
breathable diaper to reduce the heat buildup?
Good luck and please let us know what works for you.
--John