+x+xI built a lot of skis with balsa cores where you run into the same issues. I would recommend machining a pocket and fill it with glass and then some birch plywood. With the top skin this will form a micro sandwich that will securely hold the screw long term.
If you only have a single layer it will wiggle lose over time and ripp out.
And if you don't want to go that route i would recommend a 0.8mm or more thick titanal layer locally. Aluminium is much better for this application than composites as it's much tougher.
Lester, this is slightly unrelated. But on your balsa skis did you use vertically laminated strips or endgrain? I've heard vertically laminated simply doesn't hold up to shear forces, but I've been curious to hear from someone with direct experience.
I used cores from Bcomp which are engineered specifically for the application.
I used a lot of end grain balsa back in the days for kiteboards which has extremely good mechanical properties but can be a real pain in the ass to machine to thin thickness. I would not recommend using flat grain balsa and not just because of shear strength, but mainly because of peel strength and low stiffness in the out of plane axis which will promote face wrinkling at lower stress levels.
End grain balsa will absorb more resin and this is the only reason why the shear strength is better over flat grain as both directions are 45° off axis to the load direction for this specific load case.
I hope this gives you a bit of an overview. Keep us in the loop with your skis. I am really curious. Are you building competition touring skis?
I am currently in the process of building a pair of super light freetourers. target is around 1kg per ski with full size base and edges and sidewalls for a ski 1850 in length and 90mm waist. See how that goes...