Installing signs on the sides of a mason dump. Sides are built up with three 2x10's. Outside of boards are flush except for the three vertical rows of carriage bolts that were used to build the sides. Installing aluminum signs on side of boards, but not sure how to affix to 2x10's. The heads on the middle row of carriage bolts are in the way and will bow out the sign. I was going to attach with bolts and put some washers on to clear the head of the carriage bolt. Sign guy said wind will get under and rip sign off when traveling. Any suggestions? Anyone that has this type of setup? Thanks.