Wesley Cox in the Bolger mailing list had a good idea I thought was worthwhile to pass on:

I, personally, minimized the size and weight of my kit, put it and registration papers in double plastic baggies inside a length of 2" pvc pipe w/ one solid cap and one threaded with a fitted steel collar with a small ring welded to it and tied to the boat.  A semi-water proof, small, durable container.  Tied snugly to that was the spare prop.  Working on a motor while stranded on the water is SO much fun!  ... I'm a fabricator by trade so the steel collar was easy, but something could easily be fashioned with a stainless steel hose clamp or two.