I like your light bulb idea, that will probably work great for him. I don't dare turn out his backing spot at night right now because he's weak and it gets down in the 60s at night in my house, but I'll figure something out to keep him warm enough and let him sleep well.
If you've been rescuing iguanas, maybe you know how to get them to gain weight? I got this guy from a pet shop (not knowing too much) and now I realize he's super skinny and weak. I don't even dare move him to the bigger house because I'm worried it will stress him out and he's not eating and drinking on his own yet. Poor guy seems to be slowly recovering (he moves around a lot more now that he's getting proper food and not just lettuce), but I don't think he's out of the woods yet. Any advice you might have to tempt his appetite and get him strong enough to move to a bigger space would be appreciated. At least he loves to be handled, so I can give him baths and spoon-feed him. I can't think of anything else to do for him.
As it happens, I'm a college graduate and want him to be my companion for the next twenty years or so. I hope to have him free-roaming once he's a bit bigger and stronger and am currently trying to catch up and learn all I can to get him through this rough patch and on to an awesome life.