We've had this inside light cover made for our customers with 1965 and 1966 Airstreams.

It is made of strong, heat-resistant Lexan® polycarbonate, rather than the original low-grade plastic which melted in the heat. It is the only replacement available that fits on the original base using your original rotary switch and requires no trimming or modifications.  It will make your fixture look brand new.

Overall size: 13.63"L x 4.75"W x 2.00"D.
Recessed surface (with switch hole) is 1.64" from mounting surface.

This is the plastic diffuser only.  You must reuse your original base, switch and decorative aluminum insert. You may need to trim your aluminum insert by a very small amount on the ends.

We also sell a rotary switch that fits this fixture.