You could build an above floor wood framed pool lined with EDPM pool liners in any size and configeration you want and then there is no problem with accessibility limitations imposed by the size of your stairway or doors.
Most of the liners are black which is a strike against them but if you use white sand that is not likely to be a problem. You should still have good visibility from above.
You can build rectangular, square and even "L" shaped pools. It is a versatile technique for constructing large custom sized pools.
If your serious about this I have the perfect container for you. I first saw them used at a wholesaler as sumps, coral, and rock vats. Since I have used them as holding and growout systems as well as lookdown reef tanks with a suspended halide over them. Easy to plumb, many even come with threaded bulkheads built in. Best part about all of it is that they can fit side by side with no wasted space and they need no bracing what so ever.
If you want to see how they can really be used look at www.livefishdirect.com and look at their facility pictures.
Here is a link to the best ones I have found. Prices are usually about 250-300 a piece, but they will never wear out and you can actually get them into a regular doorway.
livefishdirect is awesome their facility is about 20 mins from where I live. Josh, Steve, and Rick are all top notch guys. Their facilities are all run off of geothermal water. I have been bugging them to rent me out some space to breed rays in but we haven't settled on any terms yet.