I use a 3 inch wide sheet of aluminum under each riser pipe that the fin is then screwed to. The backing strip holds the fin tightly to the riser tubes, and may improve thermal performance in some cases (see below).
Some have reported using this alternative construction instead of using the 3 inch wide backing strip:
The collector box is built with a plywood back.
Inside the box, a 1 inch thick sheet of polyiso insulation is placed on top of the plywood back.
The absorber plate sits right on top of the insulation board.
The fins are then tightly clamped to the riser tubes, and are secured in this clamped position with screws that are long enough to go through the insulation board and into the plywood backing.
I've not tried this myself, but it is reported that this method provides good clamping of the fins to the tubes.
Performance: From the small panel testing, I believe that for most homemade fins, the backing strip improves the thermal performance a bit by increasing the fin thickness where the heat flow is greatest, and by offering a little more thermal path into the riser. But, for Tom's fins, the wrap is nearly completely around the tube, and the test showed no thermal improvement for the backing strip. So, for Tom's fins (or any fin that wraps the riser well), I don't think that the backing strip adds significantly to the thermal performance -- it just becomes a way to clamp the fin to the riser and keep it tight. The alternative method described above likely does this just as well, and you may prefer to use it.
Gary November 23, 2010