"Dave Balderstone" revealed this URL to us

http://www.instructables.com/id/EW28Z3LF18DW901/

That is impressive. I like the little 9 step thing at the top of the page
that shows how he built it.

I am amazed that this would even work. Even for the short distances
traveled. I am sure it could be refined.

The metal one seemed to work better than the wood ones.

This would be the ultimate water based exercise! You wouldn't get much
distance, but it would be perfect for wind sprints!