I use envelopes from Hollinger Corp. for my 11x14 and 12x20 negs. And they make great boxes also for the negs.
Just one man's opinion. I considered the Hollinger envelope to be hard, inflexible and thick. I was concerned about the negatives in the 7x17 size. The old Hollinger box is great, storing negatives in the vertical position. However it filled with too few negatives when using the thick envelopes.
Sandy King and Robert Puckett recommended using a fold in half sheet of Light Impressions acid free, buffered, Apollo paper. I believe their Renaissance paper will work as well. I use Product no. 3141, size 14x18, folded in half and trimmed to fit the 7x17 box. You would need a different size. Naturally they were out of stock and I had to wait a long time, but the product works for me.
Since I had some Hollinger envelopes left over from the initial order I use them as subject dividers. I shoot in subject series. One subject for a book has over 100 images and has been building for two years. It takes most of a box. The second box is more assorted series and subjects.
John Powers