Maybe this will be of some interest.
I needed to replace some plastic bushings in my tractor seat. I check to see what the price would be to replace them $10.40 apiece. I bought a 2X24 inch of UHMW for $10.00.
The bushings would be 1.5 OD with a shoulder of 2inch and the ID would be a square hole of 1 inch.
Turning the OD and drilling the ID was a snap. Cutting the square hole took some thinking. So this is what I did I clamped the UHMW in the mill vice using two V blocks.
A morticing bit with out the drill was placed in the mill and used like a borch cutting out the corners. This did not work too well as the UHMW moved under the pressure of the chisel.
For the second attempt, I left the ID as drilled and cut the bushings apart and made a pot chuck to hold the 1.5 inch end. The pot chuck was split down one side.
The pot chuck was placed between the V blocks and mill vice was closed putting pressure on the pot chuck, clamping the UHMW in the pot.
The center of the ID of the UHMW bushing was found with a dial indicator than it was just a matter of moving X and Y directions 20 thousands or so and using the quill to shove the morticing chisel through the UHMW untill I had my 1 inch square hole.
The bushing with the blue was the orginal, I did leave the shoulder on the new ones some what larger than the orginal ones.