I would speak to a local Automotive Upholsterer about getting a new replacement sewn in. Even better if they can replace the plastic rear window with a glass one. The 987s soft tops have a glass rear window instead of plastic so it can be done.
Thanks Luzzo, and everyone else.
Long story short, the oil in the coolant was the real sign. The oil level sensor had dropped when I restarted the car after a drive for lunch with my wife, but I always found it a bit unreliable when warm and the dip stick looked fine... Because it was hot at that point, I hadn’t taken the cap off to look in the coolant, and the level was still ok.
Next time I went to drive it, sensor showed empty but with no puddle of oil on the floor that didn’t seem to make sense. I got some advice that it was probably the sensor at that point but obviously it wasn’t!
So I guess paying more attention to it would have been the only way to catch it any earlier, although I’m not sure how much difference to whatever the outcome will be that would have made.
Will do, and thanks.