So other than some plastic strapping what's the point? I realize it's a reuse thing, but plastic like bottles tends to get brittle quickly when left in the sun/outdoors for long periods of time. A picnic table held together would probably last a couple of years in the sun, maybe longer in the shade, but some nails or screws are going to last far longer.
I'm surprised I've never seen a DIY homebrew HDPE extruder. Take a foot or so length of iron pipe, find an auger that will fit inside, cap both ends (one will have a hole for the extrusion the other a bushing for the auger to fit through). Add on a heating element, gear reduced motor, and an Arduino to control it all. And cut the pipe for your feed opening. Technically most plastics would work, but HDPE seems to have a wide soft/liquid temperature window.