Have you checked out rosstraining.com? there is a wealth of info there regarding sandbags and shit. You can shoot him an email if you have any Qs too.
Personally, I would just get the Army duffel bag, if it is too big you can just tape the top of it down to whatever size you like. It doesn't have to be perfect, or pretty as long as it does the job. Just get some gorilla tape and you are all set.
I use my old Army duffel for sandbag training, and it has held up nicely.