Hi.
What tuning, ie. do You require a flat or a radius plane?
Way back when I tried (and failed

) slide bass, I just took a nut blank and filed the nut slots so
the top of the strings were on the same plane. Adjusted the bridge accordingly as well of course. That way there was a good contact without unnecessary pressure, and without rattles.
I'd say just shimming the existing nut most likely doesn't work well.
Regards
Sam