IIRC, the filter tray on the 23C is between negative and lens, in which case you will want an optically-planar Wratten gel filter or the like. These you cut to size. Be sure to keep them clean! Filters in the filter tray get dusty pretty fast. As an alternative, I would likely use a filter over the lens as mentioned above.
The best solution, however, may be to attenuate the light source somehow. A different bulb or some kind of ND sheet or diffuser between bulb and negative. Or, just stop down more...
Doremus