My GSO 12" Dob did not have a tube cover and as i am using my shower cap i contacted a South Australian supplier who makes Woks and Frypans by Metal spinning manufacture. I had him make the cover for me, the closest size of his mould was 370mm Dia x 33mm deep made from heavy thick wall Aluminium.
As it was a bit too large i glued in a strip of carpet (the type made for car interiors) and it fits very snug. I painted it with Hammertone finish very similar to the spotting scope mount so it looks like it was part of the original.
Very happy with the finish