Teleport. A 'From' and a 'To' so it takes up two squares on the map. It needs to be ridiculously expensive - something for when the map is getting full of weapons. It must be ridiculously slow to reload, much slower than a Rockster navigating the entire map.
The idea is one puts the 'From' base in the last square and the 'To' in the first square and any monster that occasionally make it to the end gets sent back to the start.