Having set out the basics, Rickless considers several questions one might pose for the Barrier Theory. For example, must the barrier be solid? No. Must the barrier be erected by, rather than merely used by, the rightholder? No. Must the barrier be morally permissible in the first instance? No. While Rickless's answers seem correct, I think that they raise some concerns about the Barrier Theory.
Some Questions for the Barrier Theory,
San Diego L. Rev.
Available at: https://digital.sandiego.edu/sdlr/vol44/iss4/7