A little different and serves a very specific need, but it works really well and I can confirm it's effect on handling (and rider awareness) is zero.
That red bag carries sandals. I came up with this after riding Cycle Oregon on this fix gear in 2012 and suffering foot injuries and toe infection from pulling up so hard and for so long. The sandals meant my feet were unrestrained in open air during the rest stop. (A couple of years later I removed the straps from my cycling shoes and installed laces. Problem solved.) This photo is proof that if you cannot see something and forget about it, its weight doesn't slow you down. This photo was taken on a two mile hill at the steepest point; 14.5%. So steep and hard that when I touched my forearm with the bar of soap hours later, it hurt! (I forgot to flip the wheel until it was too steep to stop and start in heavy bike traffic so I was muscling a 42-17. Really glad that being under the DT turned the sandals into no-see-ums. No-see-ums don't weigh anything.)
The issue I had to address very carefully was interference with the the wheels and drive train. Bike's a fix gear. If that bag slips into the path of either, well the drivetrain isn't stopping. Stopping the wheels rarely ends well. So I had that bag made to be a very snug fit and there is a quality toestrap around it, through a "belt loop" on the bag and between the cage and DT.