This uses 4 M4 button head cap screws but socket head might work as well. You'll need 12mm long bolts at the minimum but could likely fit up to 20mm. You'll need some small washers for the two larger holes as well as some nuts that are ideally locking. To mount this to the MGN12 you'll need 8x M3x12 SHCS as well as some M3 T-nuts and M3x8 SHCS to attach the rail to the extrusion.
The author remixed this model.
Like a couple of the other remixes of this part I changed it to use the square holes as well as M4 bolts. I also rounded some corners to make the part a bit smoother.