I had the same problem with ply spacers but realised it before flying I changed to aluminum used blue locktite, flat washers and lock washers, no trouble with loosening in 31 flights so far, also I did apply a thinned epoxy to the firewall several light coats before I mounted anything it seems to tighten up the wood pretty good.Just make sure to spread the load on each bolt with a flat washer first then lock washer and locktite. Good luck