Questions for you guys: What would be the difference between using Ian's girdle idea (linen or leather with tie points) and using a leather kidney belt with the cross straps and buckles?? Would there be historical variation, difference, or significance?? Kidney belts are more protection than support from a Roman-Greco era when Gladiators fought but seems it would work the same in principle for Crusader era maille chausses or even plate armor (depending on how you rig the suspension). It might be more rigid but certainly seems feasible. Thoughts???
The historical significance is that there's no evidence for a kidney belt in the middle ages, or anything discretely worn to protect the kidneys. Obviously things like a Coat of Plates or the fauld on a cuirass, or maille would do it, but a kidney belt is something else entirely. Nothing like that shows up in any armory inventories that I'm aware of, which is likely where they would show up if they existed since they wouldn't be a visible layer in artwork.
That being said, a kidney belt may physically work to support a pair of chausses, it's just not optimal. The shape is basically a rectangle with a swell in the middle to protect the kidneys, so it will probably tend to ride down over time. That's why a normal belt isn't that great either. The materials for the kidney belt are also not ideal. The reason for not using thick rigid materials like that are important. The thicker and more rigid the material, the more it will dig in to your hips and make wear uncomfortable. The reason for wearing a flexible, supple leather or textile is to use something that conforms naturally to the body. With increased application of downward force on something flexible and conforming, you may feel increased pressure, but you won't get edges digging in to your hip bones and skin. With any sort of thick, rigid material, over time you will get that feeling like your belt is cutting in to you. The downward force will be variable as you move around, so you don't want something that will cause you pain every time you lift your leg and it tugs on your support system.