You could buy blank stock from Brownell's and cut/drill it but you're only looking at about 2.25" base and if the Nikon sight mount (I presume it's a rail-type clamp?) is longer you'll have ejection issues. You might could shorten the clamp but you'll most likely void any warranty.

I suggest either going with a ring-mount red dot or a JP red dot (formerly Tasco Optima; DocterOptik red dot is even better so I hear) and using a stock Weaver front base.

Be aware the Weaver bases will usually require a higher front sight in order to get POA with the factory rear.