Well, as long as it's sized correctly, any rack could work, I suppose, if it lines up with the pucks and tabs on the boxes.
I have the Touratech Zega Mundos; same as the Pros but without the plastic edge protection.
The way the boxes are designed, however, it requires a tight fit between the pucks and the rack frame--the boxes snap into the frame by way of the pucks, very snuggly. The lower pucks have big fixed tabs cut into them and those tabs slide over the bottom rail of the rack. The upper pucks then slip in *snuggly* under the upper rail of the rack and the steel, tongue-type mounting tabs are turned up into the puck notches, and lock in behind the upper rack rail, where they are tightened by way of the hand screws inside the box.
This gives you a very solid mount and that snug fit is important. Without it you might get all sorts of rattling (at best) or a loose set of panniers.
So the inner dimensions of the rack railing would be very important, I imagine. If you could find a cheaper alternative that is sized correctly--for that snug fit--it might work, I suppose.
But having the Touratech rack for the 2018 ATAS myself, I can say it's designed and manufactured exceptionally well, worth the money. I don't know which bike you have, but the way the TT rack attaches to the ATAS is some very nice engineering--did away with any need to drill up through the rear fender and managed to make it a very strong, solid fit.
I have a 2016AT. I have a Touratech rack from a Ducati Multistrada that came with the panniers so I can see how they mount up, very nice and snug as you say. I guess I will sell that rack on eBay and buy a Touratech rack for the AT.
The panniers are both 38 liter. Will be a bit of imbalance since the right side will stick out further. I guess I can load heavier items on the left side?