> Has anyone considered using a picture framers mount cutter for this?  It's a
sharp knife designed to cut a 45 degree (or other) bevel in thin homogenous
material, whilst running along a straight edge or guide.

Hadn't thought of that, but it would work.  A half inch bar 1 1/4 inch
wide, with one side bevelled to 45 degrees would also work (that was the
working part of the old mat cutters I used to use) 

Mike in Woodland