On the early model Scamps with the black Hehr radius windows Scamp
used rivets to attach the windows
to the shell. I have a 85 with the black radius windows
and they are installed with rivets. I think this is how they were installed prior to rat fur.
The smaller Hehr windows from Scamp
are fairly inexpensive. I'm not sure but with an ensolite liner the wall may not be thick enough for clamp ring to hold properly.
I also have a 98 Casita
used the clamp ring and the windows are also pop riveted in.