I found this Same Cortech bag to be the best size/fit for the 800 (at least for me). And the understrap always bugged me as it came loose. The bag was never at risk but I wanted it sitting tight. My solution? I simply took a zip tie and cinched the strap/loop through one of the slots through which the poster above routed their straps. And, their solution is better than mine. But, I don't know how to sew. Yes, I'll need to replace it every once in a while when I need to remove the bag, but for me that is rare and zip strips only cost a few pennies each. The bag sits very tight and has no side to side movement.