I've attached a couple of snaps showing my DB40 wing e-tract mount (model built from a CAD laser kit I offer). Note that I used 1/16" birch ply rib doublers instead of lite ply but I also brace the rails from rib to rib between the top and bottom sheeting. Once it's all epoxied together, it is pretty solid. Of course, this is another option.
The large bent washers are just there to pull in the 4-40 blind nuts into the rails. I use 4-40 nylon bolts to secure the retracts in place.
David
Last edited by doxilia; 02-19-2017 at 10:38 AM.