The box fan should work fine for a very small GH. If both sides are open it works even better. I would place the fan in a location where it's in an opening so it draws fresh, cool air in. The same with a heat exhaust fan. Placed in an opening to pull hot air out. These locations guarantee you're moving 100% of the right air in or out and they make each other more effective when used in conjunction.
It may not be enough shade for this kind of heat, looks like 10%. Maybe in one of your normal years when there isn't some crazy monsoon baking half the country. Blowing air over the plants I would consider the most important, though I don't think heat removal is any less important. They are both necessary but my priority would be airflow if I had to pick one to do first. I say airflow because it does several things; removes heat, promotes growth, removes O2 and Brings in fresh CO2, removes moisture so mold/mildew won't thrive, causes the branches to have better elasticity so they don't become brittle and snap from heavy colas, etc... There are so many more reasons, but at the same time excess heat can stunt growth or kill them.
It doesn't take much to remove heat. A small fan blowing out from somewhere near the peak will do fine. 20% shade cloth is about perfect for the temps you are dealing with. We are in the upper 90'+ every year and it works well here. I'm not trying to pressure you but if the fans don't cut it then you'll need more options.
@kansabis has a really good point about the stone floor. Stone and concrete hold heat. That's why greenhouses generally have dirt floors, with the exception of walking paths. Dirt is one of the best insulators because it likes to stay cool and moist dirt dissipates heat very well. Being able to hold moisture is a great weapon against heat. Concrete tends to be cooler than most stone because it's porous and holds some moisture.
I don't trip as long as the person posting isn't being rude or trolling. I do that for other people too sometimes. Even though it doesn't matter to me I appreciate your consideration. I am my own worst enemy to my grow log. Somewhere back there I've got at least a full page on video games, VR, and custom gaming computers. It was the off season and I had nothing new about plants to post. So I went off, hehe.
I hope this post isn't too disjointed. I can't find my reading glasses and my eyes are starting to strain and blur.