I like the Rigidity that enclosures provide but I already have an enclosed printer. I wanted to stiffen the upper frame to dampen vibrations from the gantry motion. 

whipped these up in Tinkercad and they do seem to help quite a bit. 

I printed 2 on a plate  no need to mirror anything just flip one over wehn printed and install. They overlap at the center hole on the top rail.