Good starting idea. I think your loosing too much heat from the side, and think that might be why your HDPE sheet is getting hotter in the center (indicated by the clearness). I've successfully modified a top for my 70 gal chicken feeder with the top/center section of a 220 gal IBC tote (That I cut up for a fish breeding tank.). IT WAS A PAIN IN THE ARSE, until I realized that I wasn't heating the HDPE evenly and then was very easy. However I was only having to heat the edge of the HDPE to form a lip (to prevent water from getting inside with the chicken feed.) over the edge of the 70 gal container and was using a heat gun instead of a vacuum former, but I learned more than I'd thought about shaping HDPE. Hope you've had better success since.
Hey man excellent information I’m really needing to get into this to make one big large 4x8 piece in a single small boat hull. Right now I’m gonna start welding 1/4 inch sheets with a speed welder then in July am extrusion welder but I’m Really wanting to get in the vacuum forming