I've got a 89 Westfalia Joker High Top. I'm not sure if its the original roof light, it's an electric version operated by a spring switch above the sink unit. It's got 'TP 40' marked on it:

The problem is that the removable 'liner' part, the bit which rests on the seal is going brittle and chunks are breaking off. The outer shell looks good, but it doesn't meet the seal.

I'd like to keep the whole thing as its electric and has the mossy net, I'm looking to see if the liner part is available anywhere. Anyone ever came across one?
Cheers.