You might take a look at something with a messenger bag flap that closes with a twist lock, toggle, kiss clasp, or magnetic fastener. All of those (with the exception of the cheaper types of magnetic fastener) are secure and can be opened and closed with one hand. Making the get in/ get out operations of a shoulder bag into smooth one-handed movements takes some muscle memory development, but now that I have it I find shoulder bags to be way easier to carry than backpacks (mine's definitely on the shapeless sackpurse end of things, otherwise I'd recommend it).