Well, it depends on the kind of radiator. The common type of radiator, like TLHarrell describes, is time consuming and expensive to produce in custom sizes and shapes, but there are other possibilities as well. Not all exchange heat as well as the standard type does, but I've seen some interesting designs. It really depends on the amount of heat you want to transfer away and how much space you have to do it.
As for curved radiators, I've seen the ones used in many home AC unts curved. They also rely on a slightly different design than your usual PC watercooling radiator, with tubes going back and forth through the fins as one line, instead of several water channels in parallel going in between the fins. These can be cut into parts and segments with tubing inbetween. Those are the only "DIY" kind of radiators I can think of that are still up there in terms of effectivity.