For the best optical quality, you can use Kodak Wratten gels. Their #96 ND gels work well, and come in various densities. You have to cut them to size to fit your holder, and handle them with gloves or tweezers to prevent fingerprints.
One thing to watch out for, whatever you decide on, is the internal design of the pack. Some manufacturers produce bags that are insanely over-padded, and/or very hard to customize. Their carrying capacity is surprisingly less than specs might imply.