For vehicle emergency kits, largish first-aid kits, I've drifted toward mid-sized shoulder bags like these:

http://www.sportsmansguide.com/net/cb/2-pk-of-used-spanish-shoulder-bags.aspx?a=676347

http://www.sportsmansguide.com/net/cb/cb.aspx?a=672231&ecid=EX20D

http://www.imsplus.com/index.php?target=products&product_id=30013

I like them smaller than most 'messenger bags' but larger than a typical 'man-purse'. Something large enough to carry the basics for a couple of days. But small and light enough to ride over the shoulder without looking like your hauling a trailer. And definitely something small enough that it is obvious that I'm not carrying a laptop.

I also prefer a worn, low-tech, thrift/surplus store look in a subdued color. Something that will fail to impress and/or temp thieves.