swivel cleat install, centerboard trunk aft

[newfaner]

Thanks, KC and Roger, for help on what hardware to use for mid-boom sheeting. I went with a Harken 205 swivel cam cleat w/uprelease, a 57mm standing fiddle with becket and ratchet on the cleat (HAR2674), and a 40mm fiddle on the boom (HAR2655). A very helpful Rod at Mauri Pro Sailing set me up. I'll have a 4:1 ratio, as I understand it, and I'll be using NE Ropes bzzz line of 8mm. My plan w/a machinist friend is to not test the small rectangular flat section aft on the centerboard trunk as a screwing surface (can't tell what's been mounted there before and it's a little small) but rather create a U-shaped aluminum piece (or 3 alum pieces) that mounts to it and gives me a new, large enough platform to attach to. Am I at any risk here of drilling where I shouldn't if I attach on either side of the top of the trunk, using either screws or through bolts? What's beneath all that fiberglass?!
Dave