Just did the "black-wrap" mattebox on Saturday. Seems any time I pull out the Century .6, we need to make a blackwrap lens shade...
ND gel for filters is not likely a good idea. Lighting gels are not designed to optical quality standards. Kodak sells gel filters that we used to cut into little squares and insert into filter holders for movie cameras, so you might consider sheets of that, but lighting gels wouldn't likely hold up too well optically.
I too question the gel idea but I heard it from someone & thought I might try it for video. Good glass seems to be the way to go, but I have yet to try Gels so I don't yet know.