24" is definitely way to narrow for rays. 36" would be minimum. An adult female will have a 30"+ length with tail.
So after looking at your build, and researching a bit, I think I would do 1 of the following 2:
Material = Plywood w/ Glass
Tank Height = 24"
Tank Width = 36"
Tank Length = 72 "
Plywood w/ Glass Thickness = 3/4"
Approximate Gallons = 269
or
Tank Information:
Material = Plywood w/ Glass
Tank Height = 24"
Tank Width = 36"
Tank Length = 60 "
Plywood w/ Glass Thickness = 3/4"
Approximate Gallons = 224
Since the house is still under construction, and I have the measurements of the office but haven't really seen it. I'm just not sure if 1 brace will be enough for this.