proper mess bags are waterproof, have something to prevent the slip you're getting (sway or waist strap) and angled (at the point where the bag meets the strap) main straps so the bag remains vertical while worn even though the strap is at an angle due to the fact its wrapped around your chest. here's a list for you, these are smooth and street proven designs.

Bagaboo

BaileyWorks

Cocotte

Dank Bags

PAC Designs

Push The Envelope

ReLoad Bags

Under The Weather

Zo Bags

These vary in too many way to list but the makers listed above still make the bags themselves and will make you a bag just for you for only a few bucks more than the large name brands. I know all but bagaboo are ex or still messing, and the only reason I couldn't say about Bagaboo is cause I haven't had a chance to meet them or their friends yet, but maybe in NYC they'll show up, have a few drinks.