Dialing in both yellow and magenta will add density.

You can also place an ND filter in the filter tray, above the negative or in a below-lens filter holder. The most cost-effective filters are from Rosco (or the Lee equivalent) http://www.bhphotovideo.com/c/produc...6_Neutral.html A 0.6 will add 2 stops (4x) to the exposure time, a 0.9 OD will add 3 stops (8x) and a 1.2 will add 4 stops (16x).