I think the problem you are facing is that the surface of the bellows is not dark enough and they are narrow enough for the lens to "see" the reflection off of them. Four suggestions for you that should solve your problem.

1-Make a mask like the one you have there but put it down inside the bellows inside the second or third fold under the neg.
2-Make a type of reverse lens hood by simply wrapping a length of paper around the rear of the lens forming a tube that sticks up toward the negative.
3-You may want to get some flat black fabric paint and paint the inside of the bellows with it.
4-A better alternative to the paint would be to get some telescope flocking and apply it to the edges of the inside of the bellows.

If I had your problem I would go the flocking route. I would rather absorb the stray light than simply try to block it. Telescope flocking absorbs over 99% of the light that hits it. When you hold it in your hand it is like looking at a black hole, I kid you not. I have a ton of it and i would be happy to send you some if you need it.

edit-Looking back after having written this, I think your best bet would be to do the first thing I suggested and cover the mask with flocking. I would eat my shorts if that didn't work.