E.C.
Hi S.:
How tall/heavy is your 5YO? By "booster" are you referring to a belt-positioning booster (which just positions the lap and shoulder belts to fit the child) or a "combination" seat which has a 5-point harness and then converts to a belt-positioning booster after the child outgrows the harness by height/weight?
Assuming your 5YO is mature enough to sit properly in a belt-positioning booster (no squirming or putting the shoulder belt behind him/her), a high-back Graco TurboBooster is a reasonably-priced (often on sale for about $40 at places like Target) and reliable choice that positions the seat belt well for most children. Since it is just positioning the seat belt on the child who is using it, there is no "installation" required like there is for harnessed child seats. Just make sure that you install the screws in the arm rests -see http://carseatblog.com/?p=634 for details.
If you feel that your 5YO needs something more secure than a belt-positioning booster, a couple other options are the Graco Nautilus (forward-facing seat that can be used with 5 point harness to 65 lbs. or about 18" torso height, then converts to a belt-positioning booster similar to the Turbo Booster, retails for about $160) or the RideSafer Travel Vest (http://www.ridesafer.net, retails for about $90) . For something that's only used once or twice a year, I'd lean more towards either the Turbo Booster or the RideSafer vest. The vest is a bit more work to use than a belt-positioning booster but it has the advantage of being very portable so you could easily bring it in your luggage.