For my first Mobile Domicile, this is what I did.
I removed the back seats from my Kia Sedona van. I bought an 8-foot 4x4 and had it cut into six equal pieces. I bought a 4'x8' one-inch-thick particle board and had it cut to 6'.
I nailed the 4x4 on four corners and two in the middle of long sides. I used a twin foam mattress pad for the bedding. Five plastic tubs held all my belongings, stored under the bed.
A 410-watt inverter provided power to a fan, a portable “icebox”, and kept the phone charged as needed.