The Bolt VC-310 Speedlight or "Flash" from B&H (aka Neewer NW320 or Meike MK320 from others) is a small versatile flash that is great for more compact mirror-less cameras, but accessories such as "grids" for it are impossible to find.
This "thing" is a grid light modifier that easily and securely clips onto the flash the same way the factory included diffuser does. The grid is made of hexagons at 5mm pitch with 0.8mm walls. Thinner walls were impossible to properly slice in Cura. If you want to learn more about grids etc check out this webpage.
The STL files are available in 7.5, 10, 20, 30 and 40mm grid depths for choice of spot size. The 40mm grid may be a little excessive. Most people will want the 10 and 20mm size (see attached images taken with 10, 20 and 40mm grids using 34mm equivalent focal length @ 8ft). I tried a 5mm depth grid but it had a ugly ring of light around it. I may try to experiment with more sizes.
UPDATE Jan-4 2018:
I had previously left 2mm of space between the grid and the flash face. I've now added a gel holder design that can be placed in there (see photos). I printed a bunch and used a glue stick to attach the gel films, and then trim around it with a good pair of scissors. You could could probably just tuck one in and use this gel holder to retain it.
PETG works fairly well. I don't think PLA would be a good choice as the flash can get warm from use and the camera bag in the car could also. Obviously use neutral opaque colors.
I needed to "de-fuzz" the PETG. I cleaned out the holes with a handheld 5/32 drill bit, then used a heat gun on low to carefully melt/fuse the remaining peach fuzz.
I started with a flatbed scan of the flash diffuser for dimensions and shape and mapped the image onto a plane. I used a b-spline curve to reproduce the complex curves of one side and mirrored it. Probably overkill in retrospect. Then I created a 2-D offset sketch (manually!) to create the necessary draft angles on all sides, then shifted it 18mm higher and lofted the two. The I extruded the 2nd sketch further in height to hold the gird array plus 2mm in height.
I used the part WB primitive wizard to make a hexagon mapped to the face of the grid tube extrusion, then reverse extruded the hexagon to the desired grid depth. Then I made a shell with the "thickness" tool, then in Draft WB made a 2 column hex array, then another array to fill the grid area. The array was then clipped using the boolean overlap of the array and the grid tube extrusion.
I fused the base and grid tube extrusion and used the "thickness" tool in "pipe" settings to make a open ended shell. I then fused the grid array, shell, latches and ribs.
FreeCAD can be a little buggy, I was unable to change the depth of the grid with everything fused. If you want to change that, you will need to delete the fuse, change the gridtube and hexagon extrusion lengths, and then re-fuse. Also the fillet won't work on corners sharper than 90deg, so the back is chamfered instead.