Forgot to add that I also purchased the Brompton metro zip pouch (see below) and it can be mounted facing the rider and still allows the bike to be folded with the bag on... It's big enough for wallet, phone, keys and small snacks (like an energy bar). It even has a magnet at the base so the bag doesn't rattle on the bike.

https://curbsidecycle.com/products/b...etro-zip-pouch