These are all well and good but they strike me as being a little pricey for what they accomplish. With a little effort and a trip to the hardware store for a piece of aluminum flat stock you can get enough material to build several for about $10 or less. Fabricate yourself something along these lines:
When I originally sketched this I indicated 2" wide flat aluminum. In fact 1" wide stock or even 3/4" would probably work as well and will be less expensive.
Well, ideas abound on simple solutions to this. My buddy Scott NE1RD sent along a link to show the technique he used on the 40 Meter vertical at his new house. Not only is it economical but you don't have to drill any holes yourself!