Install the plywood backer beneath the rails to fortify the piece. Be sure to bevel your cut on the plywood to provide a tight joint.
Step 14: Construct the inside ducting using the dimensions below. The drawing calls for heat registers that are only 2.75″ x 10″ due to space constraints.
Start constructing the ducting by attaching the top and bottom to the two side rails (show below). Cut openings for the heat registers, approximately 2″ x 9″.
Mount the wall side panel and then the room side panel.
Cut the guide rails to the dimensions below and attach them to the room side backing panel.
Cut the extra panel to the dimensions described below.
The extra panel is used to create more useable heat. This panel is an afterthought. Ideally it should be a steel plate painted flat black. If you attach a wood turn knob at the bottom of the panel to hold on to, you can slide the panel up into position. Turning the knob clockwise into a matching hole will keep the panel in position. The front of the panel will increase the heated surface area and will promote air flow even without a fan.
This extra panel would greatly improve the performance of the heater.
Pictured below is the final version as it would appear in a window with a power pack on the side. The solar panel cable will need to be snaked through the ducting to connect to the power pack. They can also be run through bulkhead MC-4 connectors for a cleaner look.