The thick dowel with tape worked, I held everything inverted with my weight pushing down to force the dowel into the damper rod while I cracked the allen nut off
I'e since bought one of these, but couldn't find a 28mm one so ground a bit of an edge around it to reduce it from 30mm to 28mm. Has to be usede with long extension rods though https://www.ebay.co....j8AAOSw0UdXt0rQ
The time honoured way is to find a bolt with a head of the right size to fit the damper rod, then weld it to a bar. Or you could lock two nuts of the right size together on a piece of threaded bar.
A lot of plumbing compression fittings use a 28mm nut. I bet you could solder one to a piece of 15mm copper pipe to do the same job
Just use an impact gun on the allen bolt, then you don't need to hold the damper rod very hard.