If you've lost it, I've got a spare Aeron shim and Simmer shim I don't use. If any are compatible you can have.
Thanks for the offer Jn I'm not sure if those shims are compatible, I've cut a rubber shim in half which is working for now.
I'm sailing at Sems tonight. If you come down, then see if they fit your boom (blue corolla). My simmer boom has a clamp that looks very similar to the severne booms.