Since this is an active pickup a bridged-T filter should work fine. You can make a fixed filter with two caps and two resistors, or you can replace one of the resistors with a pot to control the level of cut.
C1 controls the high frequency slope. Make it smaller to shift the slope up, or bigger to shift the slope down:
C2 does the same thing for the low slope:
So for a deeper, wider cut make C1 smaller and C2 bigger.
To move the notch to a higher frequency reduce C1 and C2 by the same proportion.
These graphs assume the filter is being fed from a low impedance source. If the source impedance is too high, this filter will cut some treble above the notch. If the filter is on the bridge pickup only, it will also cut some treble from the neck when both are selected.
I would expect the impedance of active pickups to be low enough that neither of these things is much of a problem, but I don't know for sure.
This is about as cheap and simple as you can get. Since your pickup is too loud to begin with, I don't think an active design is worth the extra complexity, unless you really need finer control over the sound.
