Re:
Do you mean one of these?
I'm not sure 'top cap' is the best way to describe it, but this is the adjuster that sits at the top of the shock.....
In case it helps - my original 'topcap' was plastic and rounded out on the inside.
Probably a common problem considering they were marrying moving plastic and metal parts.
A motorbike fabrication shop made me a new one out of aluminium.
I'm not very engineeringly? inclined, but to try and describe it.....
It was a disc of aluminium the same diameter (50mm?) as the top of the shock.
Probably 7-8mm thick.
It had a circular recess machined into one side of it that fit perfectly over the nut that switches the suspension on and off.
It had x3 equi-angular holes drilled from the outside through to the recess.
The holes were tapped and x3 grub screws installed.
The outside (the bit you grip to turn the suspension on and off) was knurled.
The grub screws tightened onto the nut that switches the suspension on and off.
There was a countersunk hole in the centre top for the screw that initially just holds it in place - this was not required because the grub screws held it in place perfectly, but the guy who made it was just being thorough.
I would include a picture, but an irate motorist forced me off the road and the back end of the bike got all bent up, so i sold any bits that weren't damaged.
I can try and draw a picture if you need, but perhaps the ebay item is what you are after?
Or any half decent fabrication shop should be able to produce something.