Alpkit do a harness that goes on the bars and is reinforced with carbon fibre struts to make attaching e.g. a dry bag easier, but it’s quite expensive and as cookeaa shows, not really necessary.
That’s the ”Kanga” about three time the price of the dry bag you’d put in it but I can see the benefits…
I’ve dangled that airlock off the bars on all of my bikes now, best fit was on my Road bike with all the cables routed under the tape, the Picture above was taken last week on my newly built London Road with an old Tiagra shifter where the side exit gear cable had to deal with a bit of interference from the bag.
Also be aware that the webbing straps do have the potential to abrade/damage bar tape if they’re bouncing about on it so do think about positioning…
My Fixie uses a cross top lever which the dry bag can make accessing a wee bit harde, for that reason I now mount it out back on a beam rack (it’s my commuter bike)…
I might buy a couple more cheap beam racks for possible future touring/bikepacking jaunts with mates, they are actually really handy for a tenner from fleabay if you want to load up the rear without a full on pannier…
The Airlock Xtra Works OK on the MTB with wider, flatter bars (720mm) obviously.
TBH the dry bags are so cheap it’s worth buying a few in different sizes, along with some webbing/straps/bungies/velcro from ebay to experiment with load set ups, you can strap a ridiculous amount of stuff to a bike with very little adaptation or extra racks needed…