"After examining the original virtual wall, with a webcam, I was able to see that it emits infrared light from its top round transparent ring and from a small hole above the switches. The light coming from the top ring prevents the robot from colliding with the virtual wall, if it comes from its sides or from behind. The front hole emits light as a beam. This beam of light is directional and stops at the nearest obstacle, probably a real wall, preventing the robot from crossing it to the other side."
Indeed, when using the DIY version, the first thing I noticed is that the unit has to be strategically placed because the Roomba will knock into it more often than expected.
So a more robust DIY version will need to have another IR LED pointing at some kind of conical reflector mounted at the top that spreads the IR signal around the unit to prevent Roomba from running into it, just like the commercial version.
Mind you, the DIY version still works very well. It's just that some thought needs to be given to its placement so that it won't be knocked away or tipped over by the Roomba easily.