Mounts for iRobot "Virtual Wall" / Lighthouse

I have a couple of iRobot Roombas to hoover the house, but I have a few areas off-limits. The iRobot's "Virtual Wall" works really well to keep them out - if the device remains standing in its correct location. One particular problem is an Ikea chair whose legs the Roomba likes to climb onto and get stuck. Whenever I move the chair, the Virtual Wall falls over or off. The next day the Roomba mounts the chair with abandon, and will not climb down again.

To solve these issues, I have created tw