I built this just to sleep in at shows etc. and look military-ish, on an Arrows trailer, it is made of wood and ply, it's a little Tardis being bigger on the inside than it looks outside.
It has a hob and grill, sink, toilet, food storage, a seat along one side that can be a single or double bed, with storage underneath, and a catalitic heater
It is not insulated as I never bothered but that would be easy to do, At this size the window in the door is enough, but a small one in each side would help.
It has more lockers around the outside now, and we can be comfortable for a week, 8 people parties can be held in squeezy comfort using a few fold up chairs inside on rainy days!
I don't have pics of the inside but if it would help you to plan I could take a few in the next few days and post them.