I carried my Brompton with one of these for a while: http://idmybag.com/images/IDmyBag%20Stap%20Front.JPG

Cheap from many luggage stores.

Eventually I saw someone else leading their Brom along using the handlebars (which they hadn't folded down) and gave up on carrying it except for stairs.

A skatewheel conversion (google it) helped too.