Turn an adapter sleeve out of a bit of timber if youv'e only got a few to do. PSI, Craft Supplies, Wood Turningz, Packards or any of the suppliers of pen tools should have them.
Kryn
Why buy one? Make sleeves for the 7mm pilot shaft to fit whatever size tube you are using. Takes 10 minutes or less and they work just fine.
Do a good turn daily!
Don