DeWidget

The DeWidget fits on top of your stem and provides a point to mount a toptube bag (gas tank bag). As well as providing a secure mount point in the right position, the deWidget disconnects the rotation of the bars from your strap and mount point so the bag stays firmly attached without it being pulled and moved around as the bars turn. If your bag has a strap normally designed to wrap round your stem, you can add an additional strap at 90º to that one to fit to the deWidget. A lot of bags come with a strap that's long enough to cut down to make this extra strap so you don't need to buy anything extra.

There's a little bit of extra info on drJ0n's blog that you may find useful here : 

https://drj0nswanderings.wordpress.com/2017/08/23/how-to-fit-a-dewidget/