Looks like someone's a Dewalt fan..... lol. All that yellow would make me uncomfortable....
Is there a router there I'm missing? (gray case next to compressor?) Would definetly be one of the first tools I'd pick up for a build like this.
How much room do you have to work with?
As far as a shape, it seemed to me that my rays enjoyed rectangle over square. Easier for them to establish boundries and seems like they really enjoy going from one end of the tank to the other. Seemed in a square they were much less active. Could have just been my rays though.
Think a head to seperating your rays in the event of breeding, fighting, food intake observation or whatever reason, and how you will acomplish this. All of a sudden a long rectangle tank sounds a lot more appealing. Will be wise to address how to hold dividers and how to secure air lines and diffusers during the build to rather then after. Also is VERY handy to always have options to expand. IF you only need a pair of 2 inch drains then install 3" or (4) 2"ers and plug them. Overbuild by building for double what you ever think you'll need. Put drains everywhere you think bio crud will build up. You'll be glad you did. I'd also want huge drains plummed to the sewer so the only restriction as far as how fast you can do waterchanges is how fast your sewer will take it.
What are you planning on for filtration?