Take a piece of thin cardboard and place against rear beam, trace outline of top of beam onto cardboard, cut out outline and transfer form to 2" x 4". Use jigsaw to cut out form on 2 pieces of 2" x 4" about 6" long, cut a 12" piece of 2" x 4" to join the 3 pieces together forming a squared off "U". The legs of the inverted "U" should fit the rear beam, now measure in 4" from either end of the 12" piece and install two 4" uprights, you now have a mast cradle, the use of wood glue is optional as is the use of thinner wood......