I think there should be a barrier, but I'm more apt to say that workforce enforcement like Japan would work. In fact in the enforcement bill, they document how well enforcement worked in Swiss and Japan...and would work here.
I believe japan only has an illegal population of 1.5%. The barrier would be a good way of making drugs hard to get through, and would make others hesitate to get through.
I would go for a physical barrier and tough employment enforcement with teeth. With teeth, I mean severe jail time and fines.