welding tailgate shut reinforcement question question

Hey guys I'm welding shut the tailgate in my homies toyota He dint want to buy a skin cause he was to cheap anyway I got the tailgate all welded up on the outside and I want to know whats the best way to reinforce it from behind what kind of steel?Box tubing,angle Iron,etc whats the best way on reinforcing it? I know I know a skin would have been better.