I found the excellent work from benjamenjohnson on thingi but had the same as some other said where the blades didn't stay so I converted the CAD a few times to get it into F360 and added a tray for a magnet to hold the blades in place.  I also added a second cut part to ensure everything stays in place.  The magnets are 10x3mm.  I couldn't find anything over 60mm long so I had to piece some additional 5x10x3 mm ones together to span the whole width.