Until you get one you could get some small lead shot/sinkers etc. & flatten them. Next best thing to test with I'm told & you can try a variety of sizes/shapes/weights.
I have a small 0.13 gram gold nugget I take with me to help set up the GMT in different areas or to do a check with & find it a good size for that but I can only barely hear it with the SD2200D coil right on it so will be playing with some lead to test it some more.