That's just too cool. The principle...

Device stops itself by magnetically engaging mechanical stop when it approaches stop position. Stop retracts when device moves away.

must have other creative uses. I've been thinking about it in the shower (where I do my best work) but haven't yet come up with anything clever.

The need to drill a hole in the floor could be eliminated. A small hinge, one leaf of which is held to the floor with some sort of adhesive, would do the trick. The other leaf is pulled up by the magnet and interferes with the catch on the bottom edge of the door.