The best drybags for storing staff on the tramp are made by
Stearns AmphibiGear IMHO.
http://www.sailnet.com/store/item.cfm?pid=22380
http://www.sailnet.com/store/item.cfm?pid=22384
They are expencive, but tougth and have wide round solid opening with "screw-in" caps similar to the hatch caps.
Very easy to open/close underway and watertight. Bags can be inflated and used as additional flotation, backrest or racing mark.
I got two (square ones) last year, and made over 1000 miles of coastal trips this year. Never regretted spending $120 on the bags
Vladimir