I've been meaning to re-read this thread and finally got around to it. Unfortunately it was pretty quick with my kids bouncing around. I do have a few questions and I apologize in advance if these were already covered earlier.
Did you weld this with MIG, flux core, AC stick, DC stick, or what? What welder did you use specifically?
Was the original plan to just have the tank lined with the 1/2" HDPE and then attach the window to that? But you wanted the liner for extra peace of mind?
How did you "heat weld" the HDPE? I assume you used some sort of plastic welder? Which one if so? Did you ever test that seal with even a shallow fill?
And do I understand you got all this steel for free? That's crazy. Steel is so expensive right now.
Tanks is great. Glad to hear you're running a de-humidifier.