So my sister and I made a TARDIS for our cat, Cake.
Here are some pictures I took while we were building it.
It took about a week+ to build and we working on it for hours every single day. We went to Home Depot so frequently for it that multiple people in the store began to remember us, constantly asking what we were back for this time.
It cost about $160 for all of the materials (not including the materials we already had at home at our disposal)
The front door opens as well as the bottom panel on the front door which is a cat door
The inside is lined with felt and has a shelf.
All of the side panels hook together to form a stable box, and then are able to be separated to allow the TARDIS to be stacked for easy transport. The roof also separates into two pieces.