Sept 2019 Update: Though not totally "done," both the exterior and interior are ready to show off.
▶︎ The new cabin is 7 feet wide x 7 feet long and 5.5+ feet tall, and weighs somewhere in the 300-400 lbs range.
(This contrasts with my original DIY Truck Camper, which was 5' x 7' and 4.5' tall inside the cabin.)
▶︎ I built it using the "Poor Man's Fiberglass" on styrofoam technique attached to a frame of 1/2" plywood strips, glued and screwed for solid watertight construction. Cost for the wood frame was less than $100, counting glue and screws.
▶︎ The exterior is decoratively paneled with stained 2.5mm plywood.
▶︎ The aerodynamic scoop is also made from styrofoam. The full range of aerodynamic features allows it to still achieve around 20 mpg in the highway, despite increasing the frontal area by 63%, no doubt helped in part by decreasing the weight, even if by a much smaller percentage, around 5-10%.
▶︎ The camper is built as a "slide in" with tie-down hardware. As jacks are very expensive, and I don't immediately have a need to remove the from the truck, I've left that for a future project.
▶︎ The roof is the same "arched ultra-lightweight poor-man's-fiberglass popup roof" I built in 2015. Once I've installed the hardware, it will pop up towards the rear, adding about 18" to the cabin area.
▶︎ Not shown: I'm still using the same 100W solar panel from Grape Solar, and it powers a used 140aH AGM telcom battery, via a Morningstar SunSaver 20L Charge Controller.
▶︎ The cabover space is now storage.
▶︎ The interior wall is covered with an acoustic diffusor of my own design -- in geek-speak a "one-dimensional binary amplitude absorptive diffusor based on a much larger maximum-length sequence".
▶︎ The small vent is powered by a "free" 6-inch12V computer fan.
▶︎ The little Arctic Air "12V swamp cooler" is available here: https://amzn.to/2m3iIfd
Let me know what other questions you have!
-Rik