Simple and cheap to make.
Very stable.
Keeps drybag off headtube/levers
Allows free passage of cables.
Lightweight circa 70g

Drybag obviously straps into the loop!

It uses 1.5m 25mm webbing (more depending on size of drybag you intend to use), 2 HD ladderlocks, 2 side release buckles and 2 triglide buckles. Hopefully you can see from pics how it's assembled, the triglide buckles allow you to position the bag holding strap at the correct height above the crown, you could in theory sew this piece on instead, but I prefer the flexibility of using it on different bikes.
To add stability for larger bags, you can spread the straps apart on the handlebars, a couple of zipties will help keep them from slipping back in. Often gps/light brackets will have a similar effect.
Hope it's of use to someone,
Anthony