File this under 'nerd' but this was a simple request I thought many people (especially screen printers) needed but nothing exists to do it. Well, nothing I could find anyway that fits in a lateral file, so I had to make one. The strategy is just a box within a box that angles the 11" side of the sheet so it fits in the drawer.
The box is about 15" x 18" and made from an unnamed but free construction material. ;) Mark 3 equal seqments of 5" at the top and just insert another sheet of cardboard from that mark to the bottom corner.
The key dimension is the 11-3/16" angled piece. This is all just taped and glued together, which might have been easier to do before the outer box was closed up.
Add little post-it tabs with labels on each paper divider to make it easier to find and organize things.
Update: Just get a flat file on craigslist.