Some other cottage industry companies carry similar items such as MLD, Zimmerbuilt & Zpacks. If price isn't an option, these are proven true or they wouldn't sell them.
However, if you like MYOG and are half way decent at it, it wouldn't be too hard. You could make a stuff sack out of your material of choice, with a zipper at the top or along the front of the stuff sack instead of a drawstring closure. Or you could even use the drawstring closure instead of a zipper.
The trickiest part is keeping the pocket in place on the strap. First, you want install a closure system that will go around the strap, such as a strip of velcro or small webbing with a snap. That will hold down the bounce factor. Then you want to keep it from sliding down your strap. If your pack uses a sternum strap, a 2nd strip of velcro or webbing parallel to the first (one above the sternum strap and one below when in use) could work. Or, you could do something similar to the above referenced sellers and have a clip that would attach to your strap.