DougB, welcome to the boards.

When you bevel the foamcore so that the bases can "butt" together you bevel both sides inward. In other words the two sides that will but together look like and inverted V. Like so /\

This allows the two foamcore bases to slide side by side with little if any interference. If the cut on the foamcore was completely vertical. Like so ||. There is a possibility that the cut might still have a slight angle and cause the side to push out, if the angle is at the bottom of the foamcore.

I hope that all made sense. Once again welcome to our community.

I re-read your question and I think I might have answered about something completely diferent. I was talking about foamcore bases. You seem to be talking about mattboard siding.

To bevel the mattboard siding you still bevel in the same 45 degree direction. The cut on two adjacent parts will still look like this /\. When you match those two pieces perpendicular to each other you will get a straight angle of 90 degrees. An L shape. Cut the outer piece to the full size length of the outer wall. The inner wall, if you will use an interior build, will have to be cut lightly shorter. The width of that shorter piece will be the length of the outer wall minus 2 X width of the mattboard. If you don't plan on making an interior build you need not worry about cutting interior walls.