Lets see if I understand correctly. You are taking an external and dispensing with the heavy bag that came with it. I would just use two dry bags and strap them on. You could always make a small holder for water and strap that on as well.

Another option not considered: ditch the frame and make yer own with a plastic sheet (.06" thick HDPE comes to mind). Attach shoulder straps and a hip belt to the sheet, then strap everything else on just like you would with the jansport frame. I am in the middle of trying this myself. I am taking my time so it is going slowly. <img src="/forums/images/graemlins/wink.gif" alt="" />
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