NOTE: These were done to be fairly simple and serviceable. They have room for improvement as they are (see below), but I was not trying for uber precision cuts or lines, so if you want to mention I cannot cut or draw straight, I'll just agree with you.
Why white board? Because if I want it colored I can spray paint it whatever I want White makes it easy to see the marks for cutting. Oh, it also would cost me more to buy black posterboard than to spritz some paint...
Next, a touch of glue on the upright where it will bond to the top and side of the base, hold it in place, then fill in to provide support for the door and the upright. Like I said, I like hot glue for this.
Next, just add another door and another upright until you get all the way around.
I still thought it was funny when I asked if anyone minded me using proxy pods, got a hand wave & "sure, whatever" type response and then "wow" when I didn't pull up a soda can for a pod.