Yes, I had thought about the copper sheet and soldering it to the pipes, but didn't know where I could get it.
Even better, I'd think, if you could form half-circle valleys in exactly the right places in the copper sheet so it'd be in contact with the back half of the pipes. Or form nearly complete circles just big enough you have to gently tap the pipes into them.
I also used that foil-wrap insulation in mine to limit heat loss through the wood sides and back of the box. What'd be really trick is for the front of the box to have the kind of thermal glass that'd let as much heat into the box as possible but let none of it escape.
I'd also thought about a kind of glass that'd magnify/focus thin lines of light so that (more intense) light would only reach the tubing and nowhere else, but definitely have no idea who to contact to have such a sheet of glass made. And then it'd get a lot more complicated anyway because you'd have to have a (likely computer-controlled) mechanism for keeping the box correctly positioned throughout the day. Similar to the controllers that do the same with solar panels, but the controller would get its signal from a temperature sensor on top of one of the pipes. Or perhaps affixing a solar panel to the box in exactly the same plane and using a positioner already on the market. And the solar panel could run the pump.
If the servo motors of those positioners are strong enough. A 4x8 box full of water-filled copper tubing is REAL heavy.
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