If I had small wood blocks to insert which were exactly the internal width of the file, e.g. 42mm, this block would keep the 'open' end apart and the file would then be like a box and would thus sit nicely square on the shelf. The wood blocks, say 42mm x 30mm x 30mm, would be fixed on one side only, either glued or screwed.
Now, before I start making such blocks I'd like to know whether anyone has seen a commercial solution already? To save reinventing the wheel, like.
If it is not clear what I mean, I'll post a drawing tomorrow.