Cheaper, a simple locker system (nicely AirBnb theme). When renting, you get a code via text, enter it and a small locker opens up with the key. When done, you can deposit the keys in there.

You could even put up a computer next to it where you can search for immediately free rooms, pay and get the key right away, or just one friendly local guide standing around ready to answer tourist question about the city.

Put it in a convenient central location that's safe (airport, central train station).