"It's just a model," he answered sullenly, tightening a little screw. "If I'm going to be trapped here for the foreseeable future, then..." He shrugged. "I need to pass the time somehow."

Mostly true. He just left out the part about Iago disappearing, leading to his loneliness and anger, which tended to push Rex to do things that were, well, extremely dangerous for everybody around him. Like building dangerous robots with the intent to take over and/or destroy the city.

"How's the gun working out?"