I went to harbor freight and got a small plastic multipurpose box and mounted it to the back. Its water and air tight, has multiple layers of foam you can pull apart and make pockets for stuff like the tool kit, spare spark plug ect... then cover it with another layer and have you sandwiched and drink onto and still close it! I like it although my dad has a hard time getting his leg over the bike cuz his shoe hits the box.
This is what I got
https://www.harborfreight.com/2800-w...ack-63926.html