measure six times and look at the bottom side of the fiberglass to see whats underneath. I ended up having to grind out a small pvc pipe used for factory speaker wire, because it was right next to where my bolts came through the fiberglass.
You may want to cut a small piece of plywood as backing on the inside so when you tightn it down you won't crack the glass. Also helps prevent future spider cracks. Good Luck.
Great comments, read through tower builders instruction manuals as well to get additional ideas. Most post thier instruction manuals and a link to all the tower sites is at http://www.worldwakeboardcouncil.com/links.html