I've had good luck with my Motofizz medium sized tail bag.
Straps on easily, securely, and accessible from either end. More straps and strings than is necessary, but the unessential can be easily removed.
I've used it on several weeklong trips including some mild off-road with no problems.
Available from Aerostich, so a bit pricy... http://www.aerostich.com/medium-moto...-seat-bag.html