For our Pack's crossover ceremony this year, my Dad and I built this crossover bridge.  Found the directions @Colorado Scouting  where they have a pdf of the plans as well as a materials sheet.  Overall cost of project was $65, which leaves you with extra rope and extra screws.  The center part of the bridge ended up being exactly 12 planks, which is consistent with the 12 points of the Scout Law.  The bridge ends up being very portable and takes up little space for storing.  Got great reviews for it at the Crossover.