Thanks again for all of your input and suggestions.
To explain the scenario - EPA100 tonearm has a large/wide mounting base and it's going onto a plinth where the arm board position doesn't allow it to get close enough to the centre (unless I cut into the plinth). It is close to the right point and sounds good but is not optimal. My measurements show that an extra 10mm length on the headshell will get me very much in the sweet spot. I have moved the cartridge in the headshell as far along as it can go and it improved the positioning. I'm using an alignment disc template to determine where it needs to go.
I was thinking of fabricating something and what @imhififan has suggested makes it clear what could work. Obviously need to manage the weight and balance
It's certainly not optimal but I want to do something for the interim. I need a different plinth but that's not in the budget right now.
I will give this a go :) thanks