Picture is the design (as it stands) for my new sleeping quarters I will be constructing at my home in Madison. It is wider than any of my previous versions. The inside width (except for the center support) is 4 feet (1.2 meters), and the bench top is 6 feet (1.8 meters). The length is the same at 8 feet (2.4 meters), and the height our standard 33 inches (0.8 meters). The reason for the additional space is the location--an open basement. We have the room.
The top of the box has an overhang on both sides, the large one providing a work table. My box in Beloit has a 1/2 inch (13 mm) sheet of OSB for the top. I discovered it sags just a little bit. So the plan for this box is to use 3/4 inch (19 mm) flooring plywood. Because the span is greater than the 4 feet I can buy plywood, I will need two sheets. This leads to a seam. In my previous designs, I've always tried to avoid having a seam because it is an other location to leak in light. However, the width of my box in Beloit has a 4' top, and a small overhang created by screwing on a 6 inch piece of plywood. There is no seam inside the box, but the overhang is rather weak. Also, my box from my second Cedar Rapids stay was exactly 4'x4'x8', and the seam on the edges was always a problem. I had overcome this with 1"x2" boards and calk covering the seams, but it's a messy and anonying solution. The better solution is a top that overhangs. In my previous boxes, this requires a bead of calk and nothing more to keep out light. In addition, I like to have an overhang to make it easier to work.
While looking for supplies, I found a 3/4 flooring plywood with "tong and grove." This allows the seam to overlap, and I can lay a bead of calk in the grove. This should eliminate the leakage of light through the seam. I also decided to add 1/8" (3 mm) Masonite hardboard on the top of the 3/4 plywood. This should create smooth working surface.
One item I think will be a nice addition to the new design is the addition of light traps for the ventilation duct-work. The idea is to make light have to reflect as many times as possible before it can enter the box.
Above is a cut-away picture showing the cavities used in the light traps. The front side of the box is the exhaust. Air enters the trap through the top vent from inside the box. It then travels around around the maze until it reaches the vent at the bottom front. Outside light would have to make no less than six reflections in order to enter the the sleeping chamber.
The read trap is for intake. Air enters from a blower on the top and then enters the sleeping chamber through two vents. While there are not as many angles, the intake has less light to enter because it also has to enter into the fan.
One concern with designing the light traps was to make sure they did not restrict airflow. The mouth of the blower is about 7"x3", or 21 square inches. The cavities in the light trap are 3 1/2" in depth (the height of standard 2x4 board). I was careful not to have any gap shorter then 6 inches on the two paths, and 12 inches for a single path. This means there is always 42 sq. in. available for air to move—exactly twice that of the mouth of the blower. Our blower fan can move up to 2180 CFM (cubic feet per minute, 3704 cubic meters per second). So that means the velocity of air at the mouth of the fan should be about 170 mph (274 km/h), and the speed in the trap should be 90 mph (145 km/h). That's still fast, but that is a lot of air to move around. The light trap will probably limit the total airflow to much less then what the blower can do. But my current boxes all have poor exhaust opening, and plenty of air moves through them. So having a better design should only help airflow.