Surly Steamroller owners...

To avoid a saggy baggy, use the Bagman support.

For ultimate fulsome squareness, put a rectangle of stiff cardboard in the base of the bag. Not actually cardboard, that stuff you spraymount posters onto, a bit like a estate agent's boards.

Keeping it full of useful/useless stuff also helps.

Thanks, I've got the Bagman QR, I'll be out looking for some corrugated plastic.