In that circumstance, I do it like I do my Chuff-Generator. There are fiberglass shims, but I'm now using Styrene as fiberglass is too hard on the bandsaw blades. I have various thicknesses, and I just dish the first one out to conform to the motor a bit better. They're all secured with CA Adhesive (crazy glue). I just stack the up until I have the required 1mm spacing.
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