I've often thought of using those hardware store aluminium premade fence panels for a diagonal crossflow unit. Aluminium has about 2/3 the thermal conductivity of copper from memory, and I think the holes usually go right thru where each bar meets the long rails...
For the cheaper prefab cost, just make it 33 percent larger than the corresponding copper one...
Modded Aprilia Enjoy 'Race' & Enjoy 'City -10Y/o brushed Crankdrives.
Globe single spd aluminium with Q100 & 10s bottlebattery.
Dual 1900W BPM/Ezee gear hub for offroad.
900W Ezee Bakfiets / Cargotrike.
"Take the risk of thinking for yourself, much more happiness, truth, beauty, and wisdom will come to you that way." - Hitch.