I found an old Westfalia accessory catalog and there
is an isolation for the pop top canvas:

It is attached at the pop top with snap fastener and
at the lower end with straps on the rain gutter. Inside
it's air damped PVC foil, outside aluminium coated.
The isolation has to be demounted when closing
the pop top.

The price was DM 372.00 in 1989. I don't know, if it
is still available. But you can manufacture this by
yourself. A picture I like to post at vanagon.com.

Helmut
'80 Camper