Polyurethane tubing may not be what you want; generally too soft for heavy vacuum work. What happens is the walls collapse. That said, the McMaster-Carr catalog has some rather hard (95 Shore A) Ether based polyurethane tubing that is vacuum rated to 28". Its around $0.67 a foot for the 3/8"ODx1/4" ID stuff (Part number 5648K26).
Better yet, if you don't need extreme flexibility is their selection of Nylon 6 or Nylon 11 tubing. The former will handle 28.8" Hg vacuum while the latter is good till 28". Both are cheaper than the PU and work with standard compression fittings (you can use the cheap plastic ones for this) that you can find at your local BORG or plumbing supplier. The 1/4" ODx 0.190" ID is only $0.15/ft (part 5173K9).
They also have a flexible Nylon 11 tubing, 1/4" OD x 0.180" ID for $0.36/ft (part 5112K13).
Good people with amazingly speedy delivery; its just damned difficult to get one of their paper catalogs. Their online web catalog is pretty good though.
You do not need a big tube as all that is required is that the air be removed from that tube (and whatever sealed area it is connected to). Smaller tube will evacuate more quickly as there is less air there. So the variable here is the volume of your clamping area.
I recently made my own using a compressor from a refrigerator. I went to HD and bought the 1/4" nylon tubing and fittings that you use for an icemaker. It develops 22" of vacuum which is plenty for a vacuum clamp (you need more for a vacuum press). For $40, I have a vacuum clamp that works really well. When two pieces are together, it is extremely hard to pull them apart without releasing the vacuum.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.