I used TinkerCAD to make the knife blade a comb, as I was unsatisfied with the other butterfly knife combs on Thingiverse.
I think these things are even better than fidget spinners, by a long way, in terms of fiddle-ability. It is so much fun to just flip it out and flip it back in.
A few people I have showed it to have commented that it does not have a latch, which is not something I overtook, given my limited design skills.

Here is a video of it in action: https://www.youtube.com/watch?v=Jcxnzy8UAPQ&feature=youtu.be

Assembly

You will need:
2x M4 x 16 bolts
2x M3 x 16 bolts
4x M4 nuts (recommend locknuts)
2x M3 nuts

• The M4 bolts are used to fasten the blade to the handles with one M4 nut (recommended locknut) per bolt.
• It is important that the M4 bolts are not fastened too tight, otherwise the knife will not swing smoothly. Use white lithium grease (or silicone lubricant) to help it spin smoother.
• The M3 bolts are used on the other end of the handles to fasten them together with the M4 nuts as spacers between the handles. The M3 nuts are put on the end of the M3 bolts to fasten the handles together.
• The small holes on the comb/knife blade are for pins. I used some ~3mm nails that I cut down to 12mm in length, and super-glued in place

Edit: I have added an assembly diagram of sorts.

Edit: If you add M4 washers either side of the blade (which will require slightly longer bolts) it flips a lot smoother.

Note: After getting a proper balisong trainer, I realized that this needs thicker tang pins that actually work well. (Future remix maybe; I don't want to invest more time in this project) I would also recommend printing with a high infill (as much as 100%) to make it heavier so that it flips better. Using locknuts for the bolts holding the blade on would be recommended, as they tend to loosen over time, and it would make it easier to make sure it is bolted together well without it being too tight and not able to flip.