I was really happy with two products from Revelate Designs, which is a small company in Alaska that makes all sorts of different bike bags.

https://www.revelatedesigns.com/...tore.catalog/Cockpit

I have a Gas Tank in the front that works great on my road bike but the Jerry Can model might be an option for the seat/top tube. The quality seems to be very high. The straps are flush with the bag and do not stick out. The owner is super responsive and can provide recommendations based on the kind of measurements you need. If you're willing to wait he can do custom builds to fit whatever strap configurations you need. Almost all of the bags use Velcro and are thus carbon-friendly.