Tapered gibs are a lot easier to make than you'd think - the shapes makes people think that they have a compound taper but in fact they don't. The difficult bit would be cutting a matching taper in the cross-slide as Sea.dog said. One could conceivably make two tapered gibs to create a parallel assembly and fix one in place, make the other adjustable.
If you do make one, make it grossly over-length so that you have plenty to cut off. The shallow taper means that 0.1mm of difference in the thickness means 5-10mm of movement in the gib.