Do you like the Fusion 360 application?I built my own for a reefer 750xxl out of 40mm square 3mm wall. I ran some load calculations on Fusion 360 and the SWL of my finished design was somewhere in range of 15x but more importantly I got the deflection down to <0.15mm at worst.
That'll be the biggest governor of material choice. Increasing thickness over 3mm doesn't really improve deflection much, going up in box size has a far greater effect.
I'd recommend getting someone to model it for you if you can't.
This was one of my early concepts
And the real thing....
The biggest problem is the fact your tank is 48" square. When modelling mine I looked at doing a completely unsupported front or just a single offset support and iirc I needed to use 100x50mm 3mm wall on a 48" span to keep the deflection to a minimum. After looking at my under cabinet setup I decided there was really no benefit to the longer open span as I could just slide sump in from the end. The cost of making the whole stand out of 50x50 and 100x50 massively outweighed the benefits.
I have my sump in a separate area behind the tank. So I can totally have supports on the 48in runs. It’s acrylic tank so a sheet of plywood will go on it and that will be cut for the internal overflow.
It has a semi floating canopy that can be raised or lowered with an inch or two of clearance at its lowest setting so it never touches the top of the tank. The tank is a custom 90g 36x36x16. I'm sure 1x2 or 2x2 would've done the job, butI wanted the stand to "stick out" a little. I still need to wrap it with panels, but I've been too lazy about it.
