They wont be fold able in the sense of hinges and folding up, but they can be taken apart. I was able to locate some of the connectors I was referring to that may make it make more sense. I'm linking an example of a corner one. Where it narrow is what goes inside the square tube. By putting the sides on with bolts (where the hole is drilled through the tube so that it also goes through the connector inside the bolts serve the dual purpose securing the whole frame. The connectors will be tight fitting at first but if being reassembled will eventually not fit so snug, but with the bolts will hold together great. Keep in mind that you need to match connector size to the square tube size, and account for the connector being part of the size so the tubing itself will be slightly shorter than the size of the side total. It wold be preferable if the panels were cut to fit inside to avoid the supports being in odd places, but will still be safe if they are on the outside just will have a bar in the middle of the bottom and such.
https://www.zoro.com/faztek-corner-connector-3-way-series-13-13ft9250/i/G2336485/
For the connections not corners
https://www.zoro.com/faztek-tee-connector-3-way-series-13-13ft9230/i/G2747771/?recommended=true
I didnt see a 4 way, but you could use a 3 way T and then put https://www.zoro.com/8020-inc-base-connector-90-deg-black-9240/i/G0811638/?recommended=true on for the forth tube direction for that back center post.
https://www.zoro.com/faztek-corner-connector-3-way-series-13-13ft9250/i/G2336485/
For the connections not corners
https://www.zoro.com/faztek-tee-connector-3-way-series-13-13ft9230/i/G2747771/?recommended=true
I didnt see a 4 way, but you could use a 3 way T and then put https://www.zoro.com/8020-inc-base-connector-90-deg-black-9240/i/G0811638/?recommended=true on for the forth tube direction for that back center post.