When I want to be sure that a loaded pull-up will not sag, or slide down, I use a skinny ( 1" wide max ) dress type leather belt over the top of it, right at the top of my hips just above my butt cheeks. Then I put my clothes on normally over the top. When I use this set up I find I don't have to be too concerned about it sliding down no matter how active I am, or how full it gets. When I need to remove or change the pull-up, it's a lot less trouble to take off an extra belt, rather than fooling around with a onesie or something.
If you want to try this method, here are a few observations about the details of what works best in my experience. You need to put that belt on pretty tight, and the top perimeter of the pull-up fabric needs to extend above the belt by an inch or better. Because it needs to be pretty tight, it is best to use a leather one. For the buckle, you want one that is small(ish). And you don't want to put it on with the buckle on the normal centered position. I usually put it on the left side, but somewhat forward of all the way sideways. With that positioning it does not tend to dig in to your skin, or tear the fabric.
When I use this set-up, I find that I can go for 4 -6 hours of active work without worrying about it sliding down or sagging.