I owe a final update. Hopefully it benefits someone having the same problem in the future.

Two L-shaped masks made from flocking material, tucked above the first fold of the bellow provided the best solution to eliminate the stray lights. Here is a picture of the mask:

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And here is a view of the projected image from a 6x7 negative. No stray lines of light visible after putting the masks inside the bellows.

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