You don't have to be careful at all; I just push them thru the ejection port any old way (well, as long as the pointy end is facing the muzzle ), and the bolt pushes the rounds into the chamber without any problems at all. Just go ahead and experiment with the foam first; start with a basic shape that's a little too large, and sand it down by hand a little, then try it, then maybe sand it down a little more...

As far as wood goes, I was going to use the measurements in the first set of pic to create one out of plywood, and I may still do that. But these foam ones are quick and easy to make.