?Glider board/free rider type thing for MBUD stroller?

So, I just got a used Mountain Buggy Urban Duet off Craigslist for $100 (!) and now I want to get a boogie board/glider thing for it where the big kid can stand and ride.... Mountain Buggy's version (called the free rider) is $100! I can't justify spending the same amount on the board as I did the stroller!

Does anyone know if any other brands (less expensive) that work on your Mountain Buggies, or do I have to just be grateful for my bargain, and use the extra cash for the board (I know my son will LOVE it, so it'd probably be worth it anyways).

Or does anyone know if the free rider usually goes on sale anywhere/time?
Thanks!
Carolyn