Spend this morning dimpling the trim tab skin and lower spar holes. Upper spar rivet holes were then countersunk per directions. Reading the directions on riveting the lower spar to skin, i sure did not like the idea of trying to "buck" rivets with such limited access, so i came up with a design for a "Back rivet bar" to get inside the trim tab and back rivet the lower spar to skin rivets. Flame cut the bar fron 1" thick steel on my CNC burn table. Polished the end where the rivet will strike. The "Cantilevered" design allowed me to drive outside the trim tab with the rivel gun. Worked even better than i had hoped!