He made it up out of whole cloth (or got the idea from one of his Breitbard on National Enquirer buddies). Maybe ex post facto studies have popped up. But I am not aware of any serious wall proponents prior to the Trump campaign.
I mean, doesn't the very fact that he morphed from a concrete wall to steel slats worry you that he is making it up as he goes along? He wanted a concrete wall for no good reason. When people said it was a terrible idea, he asked for less terrible alternatives that he could still call his "wall". He never actually said, what's the best way to improve border security?