You can use a "Hole Saw" and cut a disk of aircraft plywood to use as the spacer. You may need to glue some 120 grit sandpaper to the face of the disk which will be next to the propeller to prevent slippage.

As long as the disk is cut from aircraft ply... there should be no problems with it not having parallel faces. If the added thickness does not prevent the propeller and spinner backplate and washer and nut from gong on correctly... it will work fine. (You MUST have 2 complete threads showing outside the nut... or if using a spinner-nut by dubro, you need at least 7 turns of thread engagement.)