The USB stick will be forgotten or lost much quicker than the phone.
https://www.amazon.com/Verbatim-8GB-Clip-Flash-Drive/dp/B00N...
Using the phone directly still seems the cleanest and most reliable way. Or maybe a combination of both, like those magnetic plugs [2] but with an integrated USB stick. Bonus points if you don't have to take it out at all (until needed) by either connecting the other magnetic part for data transfer and charging or data through USB OTG and wireless charging. One can dream... but the technology will shrink even further so who knows.
1. https://www.amazon.com/Enfain-Flash-Drives-Memory-Credit/dp/...