I like to find KISS solutions because I most certainly am the second S in KISS!

I have the same bag and have found the magnets to be surprisingly strong, but I agree that the bag can flip open when it gets caught on something while carry it slung over the shoulder.

Here is my solution. I used an old kevlar shoelace from a worn out pair of our favorite bike touring shoes. I tied it to the posts on both sides of the bag, synched down the lace adjuster so that the lace disappeared under the rim of the bag and connected a small Nite-Ize s-biner to the loop.

Works perfectly.

Here are a few photos.