I was looking for the same thing a few days ago. A search of APUG brought up this,
ULF Negative Storage Boxes from Genealogical Storage Products
I got a little lost on their web site, but the following email exchange sorted things out.
“I would like to purchase archival envelopes (possibly your L style) to hold 7x17" Ultra large format negatives. I will also need boxes to store the negative envelopes.
It would appear that your ULF Film Box - 17.75" Long x 7.75" Wide x 2" High
is the box I need. Which envelopes do you suggest to hold the negatives that will fit in this box?”
They responded, “You can find archival paper envelopes for the ULF format sizes in our web store. http://www.genealogicalstorageproduc...llafophen.html
Thank you for your interest in our products.
Hollinger Customer Service”
It all seems a bit expensive, but I put in an order for a box and envelopes and we shall see how it works out. I’ll be happy to look at other solutions if there are some.