Surly Corner bar tapers out where the stem clamps and I can’t securely clamp my handlebar bag. What product could you recommend for me to fix this?

To be more specific, I have the RouteWerks handlebar bag and it requires a mount that clamps onto the handlebar. The problem I currently have is that the Surly bar tapers out from center where the stem clamps and the mount for my bag can just barley get enough surface area to hold in place before the bar tapers out to 22.2mm. I've taken it out for a ride but when I hit bumps, my bag starts tilting downward because the clamp for the mount is slipping due to not having 100% contact. Could you guys recommend something that could remedy this?

I'd also like to add that my stem is 31.8mm and I already have to use a shim that is 25.4mm to 31.8. The edge of the shim is what the mount is able to just barely hold onto.

Here is a link to the mount: https://routewerks.cc/collections/bike-mounts/products/bike-mount-31-8mm-high