Paramount 54"W x 42"D Flat File Base
Your review: Base is inexpensive but fairly lightweight. Floor bearing surface is 3/4" wide. I elected to screw it to an additional wooden base to stop it from buckling and to spread the load on the floor. It does have to carry something like 700 pounds once two files and a top and file contents are added.
Pros: Inexpensive, fits the flat file it is designed for.
Cons: Probably too lightweight without additional support once 700 pounds goes on top. 3/4" wide bottom needs unscratchable floor or something else underneath.