I've always use a product called "zap goo". I've been using it for years to mount anything inside the tank, as well as stowage on the outside and it works great. It holds things firmly in place and I've never lost a piece of stowage off a tank but if I need to remove something it's very easy and usually doesn't even damage the paint underneath. For a long time I could only get it online, but now even my local hobby shop carries it.