I've never had anything to do with a girdle before ( I know Lurch still wears one - He's actually 5'8" and fat until he puts it on ), but the pictured one would do the job pretty well if one were to tap into the bottoms of all the main caps and secure, I reckon. After all, if I am correct, it's job is to reduce lateral movement of the mains and therefore reduce load and vibration by doing so.
I do agree that counter sinking the bolts would make the end result more asthetic, but I don't think it would improve performance in any way.
I also think that any gasket, other than a thin film of silastic, would reduce the benefit of the plate, as it would reduce the rigidity of the plate.
Just my 2.2 cents.
MaygZ