This is how I'm scanning negatives. The lightsource is an LED panel that has diffusing paper on it as well as a layer of white plastic from TAP. Then I mount the camera on the tripod with a macro stage on it to allow vertical movement. The aluminum tube is dryer vent that has been lined with flocking paper to reduce stray light. This tube also serves to hold the negative flat.
The lens used is the 35mm macro. The short focal length allows 1:2 rendering easily and can be seen here for the height needed for 120 film.
For taking pictures of black and white or slides, this works very well. For color negatives, I have to correct for the orange mask.
So far, I've used a Cokin 80A filter for good general success to correct for the film color. Recently, I was trying to scan some Fujifilm NPZ800. This substrate has a chocolate color, very difficult for me to correct with just the 80A. After several runs, I finally settled on using the 80A plus the Honl Steel Green gel. It's not a perfect inverse, but it works well enoguh.
As can be seen, the system is open to the air, so dust is an issue. Also, the flocking paper is constantly shedding fibers. This is an issue. I might eventually make another tube without paper and put a turned edge on it with a soft material to not mar the negatives.