Haven't verified these figures myself, but here's the numbers the Bat posted a while back at CF...
Upper ring dimensions:
total thickness - 9/16"
thickness of wide section - 7/16"
thickness of narrow section - 1/8"
diameter of wide section - 4"
diameter of narrow section - 3 13/16"
diameter of hole - 2 9/16" (it appears to taper a bit as it goes downward - it's around 2 1/2" - 2 17/32")
Lower plate dimensions:
total thickness - 7/8"
thickness of wide section - 7/16"
thickness of narrow section- 7/16"
diameter of wide section - 4 1/4"
diameter of narrow section - 3 13/16"
diameter of hole - 1/2"
Guldstrand use to sell these, but they're no longer available (and none left over, either).