Years ago, before buying a regulator for my Ariso head, I used ND filters, placed at the bottom of the head, above the white plexiglass piece.
I went to a local theatrical supply house, and they had sheets about 20 x 24 or so, of various gray values, used for the same purpose in lighting.
One sheet was less than $5, and I could stack them as I needed. I had to remove the bulb assembly, or I could place them above the neg. If above the neg, they have to be dust free, or a near wide open f/stop to reduce depth of field. Cheap option.