H&R has some quality adapters for the X5a available. They come in 15mm (
LINK) as well as 20mm (
LINK), have chafing which allows proper seating on of the wheel on the hub. The 20mm adapter uses special lug bolts which allow you to bolt the adapter to the hub then bolt the wheel to the spacer. The 15mm on the other hand works just like a standard spacer but with a 72.5mm hub extension instead of a 74mm hub extension.
Can't wait to see the 128s on your X!