I am in complete agreement with you now. I must have missed his size in one of your earlier posts. Your habitat is roomy enough for that sized iguana. You're doing great as far as I can tell. You're asking all the right questions.
You are thinking this through well on your own. Do you already know some things about iguanas? Your ig is lucky to have a Mom like you.
Please do put a solid lid on the habitat. Create the hot zone on the floor. That way you can mount the heater(s) onto the ceiling or any distance below the ceiling. Make the various shelves outside the direct hot zone.
Shelves require ramps of some kind. 4 inch diameter or the bigger the better round wood that is wrapped with cotton rope works very well. Wild iguanas naturally climb with their feet wrapped around their tree branch or tree trunk.
Note the frog-like position of the back feet. They splay out. A flat ladder with rungs needs to be one or two inches wider than the distance between the longest toes on his rear feet. I used to make ladders with one piece of wood on each side, and with 1/4 inch dowels for the rungs, because they looked more artsy to me. The rungs were 1/2 inch apart. The width for adults was five or six inches. Skinnier ladders make igs struggle to climb. They easily climb both styles of ramps. Even small igs can.
Sorry about this long paragraph:
Have you heard about non-contact thermometers? They take readings of the actual temperature of objects and are quick to use, making it easy to spot-check the temps in various places throughout the hab. They make it a lot easier to get the right temp for the hot zone because you get quick feedback. You point them at the target, push a button, and they read the temperature of the target. They do not have the inherent heat-gain problems that regular thermometers have. I bought my first one at Radio Shack, and it still works very well, and was inexpensive. Now I have one (inexpensive) that takes the temperature of the object that a laser beam is targeting. Be aware that some objects have much higher heat gain than others. A single piece of newspaper bent up off the substrate in the hot zone can read 145F, while the ig laying next to it reads 98F on it's skin. My adult igs once in a while liked to heat up their bodies to 105F. They would not stay there long, and would soon move away. But it sure looked like they enjoyed the high temp. Mostly they basked in the 90's as they moved closer to, and then farther away, from the center of the hot zone. It saves space when the hottest area is at the wall of the hot end. The heat declines as the ig moves away from the wall.
Auto mechanics, engineers, etc. have expensive ones that are much more accurate than any ig hab needs.
Wanting to be helpful,