Or…
Get the cheapest plastic track pump you can find (£12 from Halfords). Disassemble, cut off the foot pegs and handle. Shorten by however much you need to to fit into a pack – about 3 inches. Re-assemble and shorten the hose as appropriate. Dremel all of the sharp corners off and use some inner tube or an old grip to form a new handle.
Choice of pump is critical, but I found the cheapest Halfords one the best.
And there you have a light, high volume pump for peanuts and the satisfaction of making something ideal for the intended purpose 🙂