Rok Straps will never let you down. They're a combination of cinching action and bungee. You tighten the straps approximately to the size to get around the bag loosely, clip them around the bag, and tighten them down and the bungee portion kicks in to clamp the bag firmly to your bike. Then, make sure your excess strap is tied securely away from your wheel, and that bag isn't going anywhere. Rok Straps come in pairs for around $20.

https://www.amazon.com/ROK-Straps-ROK-10025-Black-Motorcycle/dp/B00SXJLLBW

The issue with nets is that they're unreliable in regards to tension, they don't always hold things well, and require a number of mounting connections. These guys only need to be hooked to each other under the tail section, or looped onto your passenger pillion pegs, wherever, almost anything works except your rear swingarm, lol.