I can't remember when the idea first came to my attention, but I liked the look of them and they seemed like something that should be fairly easy to bodge together a DIY version to see what happens. A bit of research turned up the various designs - some shown below. The general design is a spring suspended in tension between the back of the forks and a fixing of somekind on the downtube. All easy enough. The trickier part would be making the spring adjustable, but that could wait until I'd had it fitted and tested for a while.
The spring I used came from an old angle-poise lamp. It's quite a strong one, certainly stronger than the same springs on a newer lamp I have from Ikea. Once fitted it's obvious that the springs needed for this have to be quite powerful springs if they are to take the weight of the front wheel.
I made a small bracket from a piece of plate steel (powder coated orange helpfully) and bent it to a right angle. The idea being that this would fix behind the nut on my mudgaurd mount and provide a fixing point for one end of the spring.
For the time being I just held the other end of the spring onto the downtube with a cable tie.
It's pretty effective at keeping the front wheel from flopping to the side when parked and you can feel a difference when moving. Trouble is this was all about 3 months ago and much the same as with the chainglider, I've gotten so used to it just being there that I don't notice it doing it's job anymore. So it's still in it's cable-tied half made state :) in fact I'm not sure if I'll even bother finishing it because it does the job just fine as it is.
On a side note, whilst looking into steering stabilisers I came across 'flickstands' which are purely for keeping the wheel from flopping and look like they are more suited to sportier bikes without mudgaurds.
Edit: The bottom left seems to be a Hebie as well - from Practical cycles.