What does the wheel fix up against as you tighten the nuts?
The studs on my WH carrier are very long and you can adjust the position of the spare depending on the width of the tyre (for instance, a wheel fitted with a 7.50x16 can be mounted closer to the rear door than a wheel fitted with a 265/75x16).
On my carrier you first wind on a second set of wheel-nuts (steel wheel type) and then a large washer which acts as the back-plane for the hub to press against (these never need to be touched again unless you change the type of wheel / tyre). You then fit your spare and tighten the nuts, the wheel presses against the large washer and everything is held firmly in place.