I wasn't trying to describe stainless steel fins, more to use as the backdrop in the collector. The copper attached to the stainless would merely be heat channels to the focal point. The solar collectors are never going to be 100% efficient, I was merely pointing out the affordable materials that are compatible. Bonding aluminum and copper materials is not impossible, they just don't do it without a non-electrical conductive median, which means an insulator. Not too many electrical insulators are going to excel at heat conduction, which means you are going to lose efficiency at the bond. A sheet of stainless probably costs what he paid for the roll of aluminum. Those rolls are typically .010 or thinner material,whereas I'm talking about 26 gauge (.018) stainless. The specific gravity of aluminum is about 2.64, and the specific gravity of stainless is between 7.5 and 8.0 depending on the formula. You just reached your six times point between the two.