They fit box files and foolscap files perfectly. They have self locking bottoms so no tape required and they have handles for easy carrying. These are bigger than most of the archive boxes listed on here which makes them excellent value.
Weathertight File Box
SHOP OFFICE DEPOT archive boxes 330mm By 380mm By 260mm £12.50 - PicClick UK
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Norton Packaging
Moving Boxes: Every Box You Need for Moving
Lineco Archival Document Storage Box with Metal Edge, Acid-Free Tan Boxboard, Organize and Store Files, Prints, Photos at Home or Office, Legal-Sized Document Case 15.5 x 10.5 x 2.5 Wide
Norton Packaging
UKPS 10 Strong A4 Archive Cardboard Storage Filing Boxes With Hinged Lid + Handles Size 15 x 12 x 9 Document File Packing Packaging Office Supplies : : Home & Kitchen
USED Moving Boxes for Sale, to Your Door in Days
Large Archive Storage Cardboard Boxes Pack Of 15 Office Removals Files