I remixed the original 3.5" floppy disk construction kit to accommodate 5.25" floppies instead. Included is the .SCAD source file.
Since 5.25" floppies are, well, floppy, they may be damaged inserting the corners into the slots. You can adjust the opening by changing the "floppy_thickness" variable in the .SCAD file. 1.4mm seemed to be a good compromise. I suspect you'll want to use bad discs to begin with so this shouldn't be an issue.
For a cube, print 8 3-sided corners. For an open box, print 4 each of the 3-sided and 2-sided corners. The open box makes for a nice floppy box.
Printed without supports, rafts or brims.