Yes, you can use a magnet as a spacer. They are even sold for that specific purpose. Best would likely be a nickle plated neodymium magnet of the right thickness. Heaps are sold in all shapes and sizes on ebay. Dealextreme sell them also.
I recently purchased some RE magnets so I can perform maintenance charges to my Ni-Mh 2/3A Elite 1500mAh batteries so I can charge them in my PowerEX MH-C801D charger every so often. I don't know if 8mm RE magnets are the correct size you need, but this retailer has all different types. Anyway, here is what I have.