Bob Engelhardt wrote:
... 6 mil
polyethylene sheet ... a soldering
iron to make seams in it:
...

I had to try it - it works great! His "secret" is using a strip of
masking tape between the soldering iron and the poly. Avoiding direct
contact makes control of the penetration much easier & the iron doesn't
get melted poly all over it. My tests were short enough to just use a
piece of paper, long seams would need to use the tape.

http://home.comcast.net/~bobengelhardt/PolyWeld.jpg

This shows the front (hot) side, the back side, and the opened seam. It
was easy enough to tear if I started at one end, but with evenly
distributed force it was very strong (the 5" seam did tear, but only
with one side in a vise and both hands on the other side). This was 6
mil poly, thinner would be harder to control.

Bob