For an elastic strip to go between the PMT and the SS band of the PMT 
mounting frame, I recommend the 1/2" wide, 1/8" thick Gasket Tape
http://www.gore.com/en_xx/products/sealants/gaskets/gasket_tape_tech_info.html
as the stainless steel band of the PMT frame is 1/2" wide.

Gore Tex is a Teflon foam, and this version can be compressed 
by a factor of 2, so it should be a good cushion between the PMT glass 
and the SS band.

The price should be about $200 for a 50' roll.

It is sold thru distributors.  I dealt with the Edison, NJ branch of
http://www.atlantech1.com/


8" diameter => 2' circumference
=> enough material for 25 PMTs on a 50' roll
=> $8 per PMT for the Gore Tex.