This bike caught my eye when I first saw it at the local high school track. A gentleman had it there with his children. Though he had the dropbar version. I'm curious why you opted for the flatbar. It's a very cool looking bike for sure.

Some kind of seatpost/seat mounted apparatus would work fine for you if you're not planning to carry heavy or bulky stuff. A backpack would work fine too. This is what I do most of the time. My commute is 20 km each way. You can look at ones that have vented backs if you are concerned with sweaty back.