A bit of googling shows some British suppliers, I'm not sure on the dimensions of the sunroof but there are people still doing 34x34 for an mgb I think. Seem to be boat builders as well a lot of them...
I've had similar thoughts about fitting a proper Webasto but reckon modifying a 500 frame which you can buy new would be a simpler task if a little less secure.
Sent from my iPhone using Tapatalk