The best method I've found yet does not eliminate bubbles, but it produces a very usable and uniform cylinder of plastic:
-Cut top off of aluminum can
-Cut apart containers and clean well with soap
-Cut plastic containers into pieces which can fit into the can
-Put can(s) into toaster oven at 350 deg. F/175 deg. C
-Add plastic flakes to can
-Continue to add flakes and tamp goo until can is full
-Remove can from oven and quickly compress with fitted piston and clamp:
-Leave compressed till cool
This is the piston and can support, which I turned on the lathe. The piston is just about air-tight with the cans, though it probably doesn't need to be.
The HDPE doesn't stick to the can or the wood once it cools. The cylinder I made has been turned back to shavings in lathe experiments, but I will post a photo when I make another.
I also made a leather sheath:
Simple sheath for simple shears. Made of leather: skived, saddle stitched, burnished, and oiled.