Brian,
I built a double-Flexi several years ago and have used it since (check my system for pics). Drill the holes 1.5" from corner to center which is 1" from side to center. This leaves enough material around the connection point to be strong. This distance also works well if you put 1.75" fender washers against the shelf and under each nut. The washers then leave just a few mm of surface reveal between the washer and the edge of the shelf.
Have fun building!
Doug
I built a double-Flexi several years ago and have used it since (check my system for pics). Drill the holes 1.5" from corner to center which is 1" from side to center. This leaves enough material around the connection point to be strong. This distance also works well if you put 1.75" fender washers against the shelf and under each nut. The washers then leave just a few mm of surface reveal between the washer and the edge of the shelf.
Have fun building!
Doug