Why not just put a .022 uF (I think that is the typical value) capacitor in series with a 50K resistor? Better yet get a bunch of resistors from 100K on down until you get the tone you like. YOu could get a pack of 100Ks and keep tying them on in parallel. If the 100K is a little bassy put another one in series.

Tye the cap and resistor from the left terminal on the volume pot to ground.

Hope that helps.