I've considered making a small system. Given that 90% of the time I carry my back pack by hand, this would do the job and the wiring etc would be easy to reach.
25l tank - free from car wash plus a small case containing:
Pump - identical to the ones in my backpack, £9.50 on ebay - I'd buy three and dump them when they fail
Li-ion battery - you're going to leave the pump on at some point and you'll knacker your lead acid plus they are 1/3 the weight - £40
Analogue pump controller - £5 - £20 depending on your love of AliExpress (or not).
Vin