Question was posed: how to machine a square hole 2" deep in UHMW.
Surprisingly, a woodworking mortise machine will do the job, but 
will not clean out a square bottom.  Machine (Greenlee 228 auto
mortiser) is set for slowest feed but chisel is still in-and-out
within about 2 sec to make the 1/2" x 2" deep pocket.