the corner stand has 28" sides but can only use about a 15"square on the inside of it due to the corner triangle shape<hr></blockquote>
Damn, that is tight! This is going to be tricky. To answer your question, a sump will allow you to pull a lot of hardware out of your tank and put it in there.
In this case, you going to have to shop around or go to a plastic shop and build one yourself with sheets of plastic cut to shape.
With those dimensions I am pretty sure a 10 gallon tank won't work. A five gallon might, but I am not so sure if you will have enough room in it for your protein skimmer and anything else (heater, chemi-pure bags, floss or sponge, float valves, etc.)
Plus you probably want to be able to hold more water than a five gallon tank acting as a sump.
Therefore, my suggestion would be to go to a hardware store or Kmart or something and look at plastic containers. You might find one that will work great. Take a look at Broomer's refugium. It was made with a rubbermaid container.
Otherwise you may be forced to build one yourself. This can be intimidating, but the benefit is that you will have maximum sized sump in your stand.
Keep me posted, and good luck!
matt