I'm late seeing this, but that's a very cool idea. I'd been mulling over ways to hide a blade in a costume piece, but your solution's quite direct and practical.
How does it hold up to the weight, walking around, and adding/removing the blade?
Are you willing to share the file? My friend has a 3d printer.
I still have yet to wear it to a con so I can't really say how well it holds up there. But I can shake it pretty good and it won't drop the blade. I've also considered making a grove so I can add a rubbery material for better grip. Maybe a small dot of hot glue.