Here's a raft I made a couple years ago to navigate the toxic waters of the Chicago area when i lived there. There are a couple holes in my documentation of building this, I didn't know about instructables at the time, so if you are unclear about anything let me know and I'll try to edit it.
Step 1: Cutting the pieces.
So I found these three 4X8X2 Extruded Polystyrene sheets in a dumpster, the ends were all raggedy and busted so I cut them down to six feet long. Good luck finding your sheets in a dumpster however, I've never found this many relatively full size foam sheets anywhere ever again...sorry to say. Anyway, cut them up into one foot wide sections.