DIY Modex Plate Recipe? VPR / CBA Bass Trap

I want to build some Modex Plate style bass traps. I have seen these referred to as VPR or CBA bass traps.

Does anybody have a tested recipe I can follow? I've seen a lot of talk about the general workings but not a build diary with testing.

I'm also not sure how they should be mounted on a wall. Anything touching the steel plate will surely change the modal pattern.