UPDATE: This is probably a bad design as it eliminates the blade-length limitation function of the holder housing.
This is a Roland-style drag-knife holder mount for the Mostly Printed CNC. There is a plastic spring that presses down on the tension button at the top of the holder. I printed this in ABS. You can customize it to make the spring thicker if you want more tension, or by printing in PLA.
The strap takes M3 screws.
The unit prints in two main parts which can be glued and/or screwed together with a small wood screw (there are two join screw holes on the side). But before joining them together, insert M3 nuts in the nut traps, or it'll be much harder (trust me).