Are these corner brackets that hard-to-find?
There is this one on Amazon:
https://smile.amazon.com/dp/B071W4VQPT 
You are working with a 20x20 tile and these are 2.25" Roughly

These ones: https://smile.amazon.com/dp/B0195UGD38
They are much larger, though WAY more Expensive. 

Though you will see those Brackets. You could use something like this on the Inside with some Galv Bolts or SS Screws to make it look nicer. 
https://smile.amazon.com/dp/B07CWMSS79

I'd put two on each adjoining edge. so 2 in each corner and 2 on each side on the bottom.