I had a really nerdy post about heat transfer that I decided wasn't worth posting so I deleted it. I'm glad Stephen didn't hold back.
As my brother-in-law the mechanical engineer would say, technically it's "specific heat capacity" and not "thermal mass" if you really want to split hairs, but I don't haha.
Surface area is great (add fins to your heat sink) but even better if you can put a front on it to make a "chimney" to drive some convection air currents and make it a more active heat dump. Same way a baseboard heater works (or doesn't work if the front panel is removed).
Keeping the metal out of direct sunlight would be key, paint it white or maybe a chrome finish would reduce direct solar radiation gains that would otherwise very easily make the metal heat sink hotter than the ambient air temp. Then we'd need to figure out how to keep people from stacking them or cutting themselves on the exposed metal.....sorry to wreck a perfectly good conversation about limiters with thermodynamics.