hdpe can be cut easy on table saw
so cut it to 40-45 wide then cut 3-3.5 thick strips
cutting these """washers""" , i would make a jig with vacumholding
like 2x2 feet sized for easy handling
then laying on the sheets you could make affordable them
i mean 10-30 seconds per parts
lot preparation but a lot return
also pending on how many you going to producefor making only 20 pieces monthly simply cut the holes then with washer and screw hold each parts
then cut the outer diameter
also dont forget since they are cylindrical you also can turn them on any lathe machine