Apologies, I've only just seen this. Did you have a proper look yesterday? I can't for the life of me remember
The way mine has been done negates the shroud tube touching the barrel at all. UK Neil uses 18mm external diameter which is a snug fit into both the action block rebate and the rebate which Neil creates in the end of his stripper.The 16mm internal diameter obviously clears the barrel along it's length.
The shroud ends up held securely by it's external fitment at both ends.
I'm at Kent next Sunday. Have a proper look there. Apologies again for not seeing this sooner and if you didn't get a proper look yesterday.
PS. Just had a look at that e Bay 15mm ID tube. If you're going to have it clearing the barrel, i.e. not a 'tight' fit. Go for the 16mm Internal/18mm external. That will slip into the breech block for a nice finish.