I can't help with the outer dimensions, but a 25mm hole needs to be drilled in the armboard, in such a way that the center of the hole is located 223mm from the center of the spindle. The armboard needs to be thick enough to give good rigid support to the arm, maybe 1/2" thick, and allow the clamping of the arm to the board by the big nut that holds the arm to the arm board. The big nut on the Rega should not be tightened more than finger tight, because this will affect the sound quality. Too tight is dead, too loose doesn't hold it in place well enough. Finger tight is good.
If you are making it from acrylic, be careful, because the drill bit can "grab" the plastic at any time during the drilling and shatter it, or yank it off the drill press clamp. Use a slow enough speed to not melt the acrylic while you are drilling the hole. Personally, I'd use hardwood. That is what Teres is moving towards in all their tables and arm boards. You can still load it with lead, and the wood is easier to work with.
