Remember the tent I made for my nieces. This is how I managed to make a free standing structure to support the tent. I used plastic water pipes and fittings.
Since I couldn't find a fitting that would work out for a 3 dimensional angle, I made the corner pieces out of a regular 90 degrees elbow combined with a straight junction piece glued together.
I also made a fabric "floor" with raised edges that also maintains the bottom structure in place.
The top of the tent has channels for the pipes to be inserted into
The fabric floor and the main body of the tent are held together by snaps
Here are the pieces color coded to simplify assembly
Here is the structure by itself