I had a huge iguana that used a litter box with very few problems, except that sometimes his butt wasn't positioned over the box itself. Had to clean up the occasional messy turd, but nothing major. He "trained" himself. He was so large, I didn't have a viv that would hold him anymore. I built a series of ramps for him with lights, etc. and he started pooing in one particular corner. I put a box down with kitty litter in it. Made clean up easier.
YES! it is easy. Iguana will learn anything after you show them about 3 times. First thing in the morning, put a stopper over the shower drain and put warm water in, let is flow gently and put him in. Leave in until he goes...usually he will. If he goes somewhere else, raise your voice and tell him no. Let him know you are angry with your voice. Pick him up and put him into the shower, even if it is dry. leave him there are a few minutes to absorb the situation. iguanas think slowly, but they remember quite well and never forget. They respond well if you get on their level and give them the respect they deserve as a living creature. I put "Babe" in the shower when I get up, but he goes to the shower by himself if he has to go a second time during the day. He will also go as soon as I put him in the car and drive away, hence put him on a mat you can clean. Let me know how it goes...