The materials required to build a six bin oast are:
4' x 8' x 3/4" sheet of sanded plywood
2" x 4" x 8' stud
Enough screen or metal lathe for (6) 2' x 2' squares
A box of screws (1"-1.5")
A few longer galvanized nails
Step 1: Ripping the plywood
After ripping the planks, cut them into 24" segments. Keep the set that came from each plank together so you don't need to worry about slight differences in the width between planks when framing the trays.
Step 2: Forming the trays
Step 3: Screening the trays and reinforcing the corners
This thing doesn't need to withstand a hurricane but we want it reasonably sturdy. For the corners just rip a few strips about 3/4" wide from the scrap piece and chop them down to ~5." Use wood glue to secure them to the corners with some clamps to hold them while they dry.
Step 4: Adding the tray lips and cutting the base
The base is constructed similar to the trays but from the 2" x 4." Cut four lengths of 23 1/4" to get a final box size of 24 3/4" x 24 3/4." Drill a few good sized holes in each side of the base to allow moist air to exit from the bottom of the oast. Drill a hole in the middle of the lid wide enough for the nozzle of a hair dryer.
Step 5: Putting it all together
In practice it may make more sense to have the heat source underneath the oast. I have run it this way using the base of a dehydrator which heats and puts out a moderate air flow. When filled with more hops/bins, greater airflow will be required. The bins are the perfect size to fit a box fan. I plan to dry the bulk of my harvest by placing a box fan in the lowest tray.
Even better, if you have a temperature controller you can place the probe inside the oast and cycle the heater on/off to keep the ideal temp. Home growers have reported drying hops at 95-140 F. Commercially, they are usually dried 140-170 F but with a carefully regulated air speed. The trays closest to the heat source will dry faster so rotate them out as you dry. You can use just a couple trays or stack to your heart's content, so long as you have a powerful enough heater and fan to push the air.