I needed to work around a corner of a shed green roof I’m building in a way that couldn’t just be folded. Online sources say you need a specialized heat gun for HDPE welding (and that glues won’t adhere).

I tested a weld on a scrap:

It stuck well enough to tear the material around the joint.

The heat gun got it up to about 400⁰F (204⁰C), plenty to make it sticky but also easy to buckle, melt, and tear.

I can see that a specialized tool would be easier, but for a small, low tension/wear, non-critical area, this worked fine. I think it will keep significant water from seeping into the corner.