and not using the Topeak rack system with the special rails?

Based on a recommendation from someone here on BF, I'm looking at buying the Topeak MTX Trunk Bag DXP to transport my gear, including a laptop. But I don't have the custom Topeak rack, and I wonder just how reliably the bag attaches to a "regular" rack using the velcro straps instead of the rail system.

Please share relevant experience, if any. Thanks!