I store my 7X17" negatives in the top loading boxes from Hollinger. Some of the negatives are stored in specially made print sleeves for 7X17" negatives, and then are placed in an acid free paper I buy from Light Impressions called Renaissance paper. The paper comes in a size that when you fold it over it is almost perfect for a 7X17" negative and the Hollinger boxes. I ran out of the plastic print sleeves and don't plan to get any more because I think the negative is really better off stored in the folded paper.
You can also use Renaissance tissue, which is lighter and costs less, but I prefer the paper.
Sandy King