I can understand not putting him outside unrestrained. My friend lost a nice green iguana that way. You may have tried this already, but there are sellers of reptile harnesses on Etsy, of all places, that have adjustables. They are more custom than what you can find in a pet store so maybe they have or can make something to fit his unique dimensions securely. He looks pretty hardy despite his amputated situation. Does he have a super chill personality?