there are some daypacks with a tensioned mesh back panel . Im not in sales, i dont know brand names.

but for the place you are, Equatorial, Klick Fix has a backpack you hang on a seat post bracket while you ride,

Then it pops off to put it on your back when you are off the bike. IE its off your Back when on the bike.

the seat post mount is usable with many Klick-Fix products + Ortlieb handlebar bags ..