Pete, your local hardware should carry small pieces of brass stock in gauges all the way down to a few thousandths. (Ace Hardware does, if you have one up your way) It could easily be trimmed to size and secured with some double-sided adhesive to the bottom of your nut. Thin piece of wood veneer would do the same job... just depends on what you want.
I have some pretty thin gauge brass in my shop, would be glad to send you some if it'd help.
*if you haven't already, check your neck relief to make sure it hasn't acclimated for Winter. Mine have, but they go the other way in the dry cold season.
