George's description is absolutely correct. Shrink tubing has been used in the Electronics business for years, starting out in the Space program. It isn't often seen in home hobby or automotive stores in the larger diameters - they usually have something less than 1/2" in diameter and typically 4 inches long (a 'la Radio Shack or Home Depot).
I was looking for a roll of tubing about 3/4" Inner Diameter BEFORE shrinking and much longer to cover the length of the bow with one length, and I found some 3M FP301 Heat Shrinkable Tubing at a local electrical supply house (the places where electricians buy their supplies). It is 3/4" ID and the roll is 25 feet long and costs about 80 cents per linear foot. You need a good heat gun for this particular stuff - the minimum shrink temperature is 200 degrees F, although there are other versions that shrink at a lower temp, I just didn't happen to find them and just used a LOT of heat from my gun.
I would NOT recommend using a propane torch to shrink this stuff - the torch temp is not all that controllable and you often will burn a hole right through the tubing 8>(
I cut it an inch longer than each bow end (although it doesn't shrink in LENGTH when heated, only in diameter) and then trimmed it to 1/8" longer than the bow after shrink, and the holes for the mounting bolts are easily cut out with an Xacto knife or something.
BTW: Other uses I've successfully tried: Covering the length of my wiring harness, and to nicely weatherproof an electrical connection (like in a wheel well for tail lights) I slip on a little larger diameter piece, make the light connections (either slip-ons or soldered), apply some clear silicon caulk to the connections, slide the shrink tube over the caulked connections and then shrink it - the connection is then impervious to ANY moisture. Later, you can still more-or-less easily get the tube and caulk off if need be.
Gordon
Now all I have to do is get a new top....Arrrrrggghhhh!