Here's 3 of the 4 stages of putting the back frame together.
1) The 2x6s were attached with 3" and 2.5" deck screws. We had to rip the top board to make it provide the correct gap above the window. Other than that the cuts were just for length.
2) Window taped with painter's tape to keep the Rubber Coat and Dow 795 off of the window. I like the wide tape as it gives a better margin for error.
3) Six tubes of Dow 795 were used to fill the gap between the 2x6 and the window. The 2x6s were flush mounted against the plywood sheathing and overlapped the rubber gasket providing compression on it.
Finished the first coat of Permadri Rubber Coat on the frame after taping up newspaper to cover the window. The Rubber Coat has kept very well being stored inside. After over a year of sitting close up the water separated from the other materials, but went back together rather easily with some stirring. I'd suggest having a drill powered paddle stirrer available for this task though. Two more coats then a week to cure then filling time.