I just got put on oxygen and need to carry an O2 bottle with me when I go out and about. Would like a back pack instead of the shoulder bag they gave me. Any recommendations? Also, how do I know what size bottles I have? I saw M6/M9 but that doesn't mean anything to me yet. The bottles are about 14 inches around, 14 inches tall, and 4 1/2 inches in diameter. They weight roughly 7 lbs. Any help is appreciated.