First off, wow, great attention to detail. You have produced some quality pieces and inspired alot of people with your work and attention to detail.
I also have some experience with HDPE materials, but from a different perspective. I used to work for a company that installed rolls of HDPE liner as well as HDPE piping in landfills, secondary containment applications, etc.
We used a process similar as above to "weld" our pipe in the field. If you ever try to this method, be aware that you will end up with a seam (from material "pushing" out). It looks similar to the attached picture, although not identical. The attached picture shows an extruded weld. Again, they look similar, but not identical. The extrusion process is more typically used in detail work such as constructing an HDPE box, creating boot sleeves for gas vent pipes (when working with liner), etc.
I just wanted to make you aware of the seam in case you tried something like this. I haven't tried to weld 2 flat pieces together like you are doing, but I think you would get a seam of some sort, which would need accounted for in your measurements.
On a side note, have you noticed any contraction/expansion with the HDPE blocks? When doing a liner job, during the day when temps reached near 100 degrees, we may have 2-3' tall wrinkles. At night when it cooled, the liner would be tight again. Granted, you probably aren't seeing exposure to sunlight and temps in those extremes, but being in the garage, I would be curious if they are "moving" any.