Nice “short form” Instructable from user mfoster, who cuts up milk jugs and cleans and dries the pieces before heating them to 350°F, loose, in a pot of canola oil. The temperature is monitored with a candy thermometer. The HDPE pieces soften into a clear “gel” at this temperature, but do not come apart, and are removed from the hot oil with a slotted spoon. They are transferred to a simple press made from a few scraps of lumber, and the press is closed and operated using a set of three manual clamps. The cooled blocks can be shaped with normal woodworking tools.

Making Blocks out of HDPE milk jugs