Just seal the plastic back plate with a good silicone gasket maker, and make sure you use some flat washers under the heads. They don't need to be super torqued either, especially if you get some of the gasket maker on the screw threads so it acts like thread locker. Do that properly once, and unless you crash a lot, you can forget about it for a long time. If you are using rear needle valves, try and find some that use the metal bracket in place of the flat washers there.
Type at you later,
Dan McEntee