When making the wooden hinges the sizes were not predetermined but measured as I was constructing it. Each monitor was 520 mm wide. For the side hinge pieces I made them 260 mm plus twice the width of the wood ( 380 mm). I measured and marked the line with a square again before clamping the piece in the vice. Using a tenon saw I cut the piece to size. This design requires 4 parts like this.
When the parts were cut to size I used the metal bracket adapted from the monitor stand to mark where holes have to be cut for the bolts. I used the drill with an 8 mm bit in it again.
Attaching the wooden hinge pieces to the adapted bracket is done in a certain order. The bolt goes into the wood with a washer on top, then metal bracket is placed on the washer with a nut to hold it in place. By tightening up the nut you can press and secure the coach bolt into the wood. On top of the nut I put another washer then another hinge piece with a washer and finally a wing nut to secure it in place.
With the hinge section secure I set up all three monitors to gauge an angle and required length of the main centre mount for the hinge parts. As you can see in the last picture I determined the size of this by placing a rule in place of the wood. From centre pivot point to outside hinge pivot was 295 mm. I doubled this and added the with of the wood (650 mm) for the size. 2 pieces this size are required.