How loose is loose and intake or exhaust?
You'll only need a bit of thickness. Idea won't work, something that thin will move around to tear up motor. No way to positively locate it to not do so.
You change bucket, they are different thicknesses.
To put shim underneath, the bottom of bucket and top of retainer must be configured to hold shim tightly in place. The shim is hard and must be a certain thickness to overcome the brittleness of the hardness, can't use just a few thousandths thick.