I used a small tungsten bar with a handle fabricated from some scrap 1/4" masonite (thin plywood or other stiff material will do). I taped the bar onto the handle with electrical tape.
The side skin was protected from the inside using some thin cardboard from a notepad and some thin high density foam.
The bucking bar/handle was pushed up from the bottom edge of the side plate; an inspection mirror and flashlight are handy to check bar postion.
This is best done with a bucking partner since the bucker needs to use some finger leverage on the back of the handle/bar with one hand and stabilize the position of the bar with the other hand. The foam padding worked well to protect the knuckles on the bucking bar hand.
The hardest part was finding a bar/handle configuration for each rivet; some re-taping/shifting of the bar was needed to get the best angle for each rivet.
Good luck,
Mike