Right! decided to get brave and break out the cutting tools
Looking at the plastic bits, it seemed perfectly possible to cut a slot for the Teins:
I cut a slot out and gradually widened it until it fitted the damper. Hard to get a decent edge, so the final trim was a ruler, stanley knife and patience job.
I've done one, did end up scuffing it in a couple of places, but doesn't look too bad. Might try Stans black wax crayon trick, see if that helps.