If you're worried about it leaking, use a RC boat/plane fuel tank with a brass "clunk" pick up. I've used these before and had no leakage problems. They have a breather tube/vent that you can take off an secure elsewhere.
You should only have an inch or so of expansion on the hottest of days, assuming your cooling system is in good shape.
Hottest off days was refering to odd days in mi July/August when the entusiasm/ability equation gets a little tweaked and yours truley used to sit in the holding paddock with a cooking engine, or getting red flagged part through a run and having to turn the car around in short order and do it all again. Water was never a problem with a well bled system and this system of expansion tank.
Steve,
Clunk refers to the weighted brass pick up (probably from the noise it makes) on a flexible tube that means it'll pick up the dregs of fluid, no matter where the G forces are pushing it in the tank.