Two ways of dealing with this:

First method, making the hole in the tube holder narrower

  1. Clamp the tube holder in a vice with the hope facing up (or ask some one to hold it for you)
  2. Use a nail and hammer to make little dells in the surface of the hole. Each dell will create a “crater” that stick out, making the effective diameter of the hole smaller, so more friction.
  3. More craters = more friction.

Second method, making the tube wider

  1. Sand a small section of the open end of the tube, approximately as long as the hole in the tubeholder, a few mm wide. When you have a small  hole in the tube (changed that some time ago to no  hole anymore needed) sand on the opposite side of the tube,
  2.  Apply a very thin layer of 2 component epoxy glue on the sanded part.
  3. Let dry until it is really hard and non sticky. Try to fit in the tube holder
  4. Too wide, too much friction? Sand it a little. Not enough friction? Repeat 2 and 3.