In message , Franz Heymann
writes
I am mean that lightweight double-skinned stuff which can be cut with
a pocket knife. It has an overall thickness of about 5 or 6 mm. It
is translucent, but not transparent.

Some grades are almost transparent. And if you want it to stay that way
be sure to assemble it with the UV stabilised side facing outwards.

Which urgler would like to report on its usefulness for making
"rectangular cloches", or small portable frames?

I have one outside. Two sheets clamped on a wooden frame. Actually it is
made from the 1cm thick grade intended for conservatories from my old
greenhouse. The 6mm stuff is a bit weak to be self supporting for large
cloches. The only thing to watch is if the wind gets under it.

What does one use for gluing two pieces together?

You don't glue it reliably with any strength. But you can bolt it to a
frame or to itself with suitable plastic washers to spread the load.
Clear silicone rubber sealant can be useful for weather proofing.

The 6mm grade you mention is ideal for small opening roof vents on a
greenhouse since it is indestructible if the wind grabs hold of it
(unlike glass).

I intend to build my next greenhouse entirely out of twinwall
polycarbonate.

Regards,
--
Martin Brown