Marp68,
What about making a shock extender with a sleeve, i.e a thin wall tube that is a slip fit around the shocks lower tube. Basically shaped the same as the shock, just big enough to slip over it. Then the bolt attaching the shock to the extender could also be slip fit, and just TIG welded at the ends. This should not create too much heat if done carefully. Then you could use a proper rubber bushing in the eye of the extender.
This solution would look much neater imo.
But like I have mentioned before, you would be better off with a longer shock of a known brand like Ohlins, WP, Racetech etc.