Is a better way to store mounted 645 and 6x7 slides?

I have been using card file boxes from Office Max/Depot which will hold ~ 100 cardboard mounted (Blair Mounts) 6x7's or ~ 50 in Gepe mounts. But I've always wondered about the archive quality of the inexpensive (<$5) Office Max/Depot file boxes. Is there a better storage alternative available? I have several thousand mounted slides. The 6x7 mounts are 85x85mm. There are Print File pages for the 35mm slides but nothing that I have found in the past for the 645 and 67.