I use a waterproof
Otter Box 3500. I store it in the cavernous zipper pocket on my mesh boat bag. It seems to be just the right size to fit all of my save-a-dive stuff.
My kit contains: a small adjustable wrench, needlenose pliers, one 11/16 fixed wrench, thin wrench set, allen wrench set, two screwdrivers (Phillips and flathead), Schrader valve tool, power inflater assembly tool, i.p. gauge, zipties, q-tips, exam glove, short length of bicycle innertube, CO2 cartridge, spare nuts & bolts, tire inflater adapter, some bungee cord, o-rings for everything I'd want to service in the field, o-ring picks, O2-compatible lube, port plugs, yoke insert for DIN/yoke convertible tank valve, 3 kinds of HP spools, and a spare mouthpiece. My DIN-to-yoke spin-on adapter is stashed in a ziploc bag in another pocket of the dive bag.
The Otter Box 3500 would fit in your backpack. Loaded with everything inside it feels like a 3-lb. brick.
Hope this helps...