I've been thinking about the ways to build a custom shell. Since I don't have any interest in welding, I've been looking for odd ideas that might work. I discovered that a 2 horse bumper pull trailer is just about the right size for us.
Pros: Available with AL frame and skin, fiberglass roof, insulated, should be plenty strong. Might be able to use the sub frame as the basis for a 3 point mount. Windows have bars. Size is right.
Cons: Weight - empty around 3,000 lbs. Any "conversion" would involved pulling the axle and wheels, so there is some weight savings there. Price- $15k-$18k looks about average for a new one.
My wife gave me a funny look when I suggested this.