To get the washers in place I would try a small line and contact cemment. Use a small artist brush to apply some glue through the hole in the beam. Dab a little on the washer. Tie a knot in the middle of a long enough piece of line to be able to tie the ends of the line together. This is done so you can pull the knot away from the washer. Let the glue dry and rivet. If the beam has some curve you need to match the washer to that curve.