Lezyne Road Caddy under-seat bag. It's barely noticeable. No wobble or waggle. But it's very minimal. I can't squeeze in a CO2 cartridge and Planet Bike Red Zeppelin inflator, alongside a Conti race light tube, tire lever, flat multitool and Lezyne's flat self-sticking patch kit in an envelope pouch. So I still carry a mini-pump and/or CO2 inflator in my jersey pocket.
Arundel bags are similar and a little larger, enough to hold CO2 kits. I haven't used one but a friend has one and it doesn't appear to wobble around. I know he didn't like the Serfas Speed Bag, which I use on two bikes. The Arundel Dual and Tubi bags are intended to mount sideways so they're more visible than the minimalist Lezyne Road Caddy.
There's also the
Lezyne Flow Storage Cage, a water bottle cage with a small hidden compartment and straps to hold a couple of CO2 cartridges, exposed and visible. It's listed at $60 retail on Lezyne's site, but available for $20 from Amazon. Probably won't suit some minimalists who don't want visible doodads. But possibly handy for those of us who don't have support vehicles accompanying us on rides.