Thanks Simon, I was considering a solid tube spacer, as your washer idea would do the same but I'm going down the JD suggested route of a solid base. I'm using a piece of phenolic hardwood plywood, as used by MSA, phenolic just means its made up of a 2 part phenol, we used to call it resorcinal, glue which makes it incredibly solid!
Thanks to both of you and JD for the help. Cheers

