I finished up my 8' x 18' solar kiln early this summer and ran my first load of 3"x6" western red cedar through it with excellent results. This is a modified VA Tech design.
I built the kiln with four bi-fold doors spanning 16 feet. To my knowledge, this is a unique design on kiln doors. Sliding barn door hardware supports the center corners of the innermost two doors. I guesstimate each of the four doors weighs over 100 pounds. There is absolutely no sag and the doors will easily slide open with one finger. Cane bolts on the inside secure them closed.
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The four ~4 ft. x 7 ft. door sections (actually more like wall sections) were constructed in the shop with 2x4 lumber dried in the shop, trued on the jointer, built flat and squared, insulated with fiberglass and sandwiched with 1/2 cdx plywood glued and screwed to the framing - very strong with little potential for sag or warp.
The two sets of doors were hung using three 12" barn door hinges at each of the sides (with corresponding blocking built inside the doors) and three 4" door hinges in the centers. They were hung with 1/4" gap all around, at the most, and then T111 siding was screwed on slightly oversized to the outside to cover the gap top, bottom and between the center of the bi-folds. A 1/2" inch vertical gap in the very center of the two sections was necessary for clearance when opening and closing. A 1x4 was screwed to the back of one door on the inside to close the gap.
The doors fit tight enough that any further weather stripping seems unnecessary and the short roof on the back helps keep most of the weather off the doors. The cane bolts on the inside snug them up well against the jamb and make it impossible for anyone to be shut inside while also allowing the building to be secured from the outside with a single lock on the access door.
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The bi-fold doors limit the 16' opening to an effective 13'+ foot opening which is fine for my purposes. I loaded the kiln with mostly 12' and 13' 12/4 cedar with no access issues.
The kiln is roughly 7' by 17' inside with a 12/12 collector roof, or roughly equal to my 47 degree latitude. Glazing is triple wall polycarbonate (double cell) on 2x6 rafters 2 ft. OC with 1x4 horizontal skip boards on top at 3 ft. OC for securing the polycarbonate panels. Black metal roofing is screwed onto the rafters underneath as a collector.
Four solar powered fans blow out of the plenum (horizontal plywood extends from bottom of fan shroud forward to rafters to create a triangle plenum) down across the roofing and out at the front of the kiln. Air recirculates back through the lumber. Floor is 2x8 and walls 2x4 with fiberglass insulation. A narrow insulated 24" access door is on the side. The bottom of the fan plenum is just over my height to allow easy access and no bumps on my head.
All interior walls are 1/2 cdx ply over plastic vapor barrier and insulation. All joints, holes, etc. we're caulked prior to painting for a good seal. Interior walls were painted with 2 coats of aluminum paint and then flat black (in case I ever want to try the kiln without the metal roof panels) Floor is 1 inch signboard ply (has waterproof resin coating) over 2x8 on 16 inch Centers. Floor was painted with 4 coats of aluminum paint. I might put on some type of porch paint or rubberized coating on the floor in the future depending on wear and water absorption.
Using the metal roofing under the rafters has the added benefit of being able to "turn off" the heating of the kiln, or adjust it, by turning off one or all fans. This is perfect for my use since I'm using the kiln for lumber storage (and there is never enough of that) when I'm not drying. I pull out enough sticks for weeks of work and move them to the shop for acclimation. When stock in the kiln runs low, I'll pull out the remaining cedar, move it to the shop and restock with a new load for drying.
I was able to take 1100 board feet of air-dried western red cedar with a moisture content of 15 percent (at the most) down to 7 percent in 8 weeks. I had to slow down the drying (by turning off some of the fans) when the differential MC in the center of the wood and the outside became too great. I didn't want to create internal stresses in the wood, avoiding case hardening is one of the primary benefits of solar kilns. But these were full 3" sticks so slow was the way to go. I suspect 1" cedar could have been dried in a good week of full sun.
I estimate the cost difference between air-dried cedar and kiln dried (if you can find it at this thickness) paid for the cost of construction with the first load and I was able to control the drying myself in the process.
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