This is a very nice and smooth balisong, use M4 screws all around. Be sure to use locking nuts.
At the bottom of each handle, use a screw, 2 normal nuts and finish it off with a locking nut (one nut goes in the handle, that's why the M4 nut shape is there). The handles will be nice and weighted.
This version has a printed center pin on the blade! It will react like a real balisong :)
I print the handles FLAT on the bed with the holes up. And between the handle I use support. I do this so the holes will be nice and smooth (I don't care about the inside being supersmooth you don't see that).
I printed at .2mm layer height, 60% infill to make it nice and strong!
Check it out: https://www.youtube.com/watch?v=FOcKPevliAE