Stefan: You are correct in that the size of the hole must be large enough to allow the tapered pins to be parallel with the 1/8" spacer between them. Keeping this in mind you can use any diameter parent material you have handy.
The pins can be any length you wish from 2" to 5 or 6". Like Richard said, use the best material you can get. Mild steel = short pins.
Thanks Richard K for getting this ball rolling. Always pleased with your input. Any input will be greatly appreciated.
Jere