Mmm....if it does feel like it might not be uniform for the length of the compression stroke, I'd say a precision machining shop would be the best port of call, Rhys.
Maybe NickG could do something? I think he could also actually make a comp tube from scratch, maybe? And, if so, to reduce the cost, I wonder if he would do an exchange on your current comp tube, as he could most likely use that as a basis for a narrow bore conversion for another project?