chrispy, this didn't work too well for me but it did help a little, I certainly will at lest try to xstep occasionally.
That doesn't mean it won't work for you if you can make one.
All it is is the foam from the inside of one of those 4 inch thick foam mattresses, I found it on the side of the road in a pile of rubbish and for years I used it as a car door protector on the wall of the garage.
One day I managed to get my hands on a 3/4 inch thick plank around 7 ft long and 11.5 inches wide.
I took the material covering off the mattress, roughly cut it lengthways about 18 inches wide, then attached it to the bottom of the plank with a few 1/2 inch wood screws
Basically, what I was looking for was just a narrow area that forced me to make sure my feet always stayed along the centre line, and an unstable platform that forced me to concentrate on my balance.
Depending on how big your steps are, it can be 3 to 5 steps from one end to the other. The faster you go, the more difficult it becomes and the more you must concentrate to keep balanced and stepping in a straight line.