The usual thing to do is just get some sheet copper, solder it to the pipes, and paint that, and the pipes, flat black. Then set it loosely in the box and insulate the box any handy way.
Originally i had thought of covering iron with reflective foil to sort of focus heat onto pipe in each groove but then decided it would maybe just reflect mostly out of box!
Why bother? A little foil or shiny aluminum or chrome plate around the sides bounces any off-angle light onto the black copper sheet as well. All you need to do is get the light absorbed once by the black stuff.
Once the light is absorbed it re-radiates in the near infrared and the glass bounces essentially all of it back to be re-absorbed. So the only way out for most of the heat is conduction through the glass (doulbe-glazed), the box walls (heavily insulated) or the coolant in the pipes (what you want).
When you're just after the heat a black-plate collector is very efficient. Focusing elements are for when you want high (above boiling point) temperatures as well - say for powering heat engines, or making process steam for cooking. At hot water / air heating temperatures they're a lot of work for no gain.
would aluminium react with copper when hot?
Aluminum or iron would react with copper when WET. And you'll get some moisture in there. So it's good to keep it all one metal.
Another problem is differential expansion. This stuff is going to get really hot in the day and cool off at night. If you use different metals with different expansion coefficients it's going to bend - very strongly. This might actually make it break out through the window or break the box apart if you didn't give it enough room.
Copper conducts heat very well, and takes solder well (which also conducts heat). And solder/copper doesn't tend to break from temperature cycling - as long as it stays below the solder's softening point, which is far above boiling. So the copper-pipe/copper-plate system is good.
Your pipes look kinda fat. Good if you are going to place this below the heat storage tank and depend on convection rather than a pump to circulate the fluid. But if you're going to use a pump you might want to think about using smaller pipes in your next model to save some bux. Water (and water-based coolants) carry a LOT of heat with a little flow.
Also: If you're not going to pressurize the system (for instance, by hooking it directly into your hot water plumbing) or let it freeze, you might get away with refrigeration tubing rather than water pipe, for another saving.
