I designed this for a custom board, so I have included the .step file for easier modification. 

Uses M3 threaded inserts to hold the circuit boards and some magnets to hold the backplate on.

The front panel is glued. 

I have found that some nixie tubes might be a tighter fit/too large, so the front panel tube cutouts might have to be opened up a bit depending on how out of tolerance your nixies are.

see 6 digit version here