Had a post a while back with a link to a Garage Journal forum thread where a guy did some great tool organization by milling HDPE blocks. Some info on DIY HDPE blocks.
http://www.instructables.com/id/HDPE-Blocks-From-Plastic-Bottles/
And machines for working with plastic (shredding, extruding, casting, etc.).
https://preciousplastic.com/en/machines.html
Also read some things about using HDPE as a filament for 3D printing. Not as easy as PLA or ABS, but sounds like it is do-able.