When fixing a cleat on a spar, too many holes in a small area will weaken the spar. Always drill as few holes as possible. Mark positions of all holes accurately before letting rip with a drill. Self-tapping screws are quick and easy to remove.
Pop rivets are useful where screw holes in spars are stripped or oversize.
For extra strength, use a marine adhesive as well as bolts. Once the resin is set, just bend the card to crack off any surplus. Seal holes, or slots for sheave boxes with varnish or epoxy to stop water penetration into the laminate. Where the slot is in wood, it will stop discolouration. Before cutting a bolt to length, thread on the nut, then cut off the surplus and file off any sharp edges.