wow - going to be a tight fit. i'd go to target or wal-mart or something and see if you can find a food grade plastic container to use as a sump. you may also be able to find a 15gallon High tank or something to fit in that space.
Actually, you could empty the water temporarily into a container if you were going to get a taller stand or change stands. Hmmm....I agree with MattSeattle in regards to finding a food grade plastic container to serve as your sump if you are unable to find a tank to suit your space.
also don't forget you need an overflow for your tank as well as a return pump. also you need to calculate how much water is going to drain out of your tank when you turn off the pump or a power outage. your sump needs to be able to hold that water plus the normal amount of water. my 55 gallon drains about 3 gallons of water out when i shut off the main pump...maybe a little more because i've never calculated it to an exact measurement. total my sump holds about 20 gallons of water.
I guess I am curious why you are trying so hard to fit a sump somewhere. Do you not have enough room for all the necessary equipment or just looking for additional water volume?
I ask because contrary to popular belief you can have a sucessful tank without a sump. I have had mine running for almost 3 years without a sump. Are there drawbacks...Yes. But I don't know that forcing a configuration that is less than optimal is a better solution for you. Just a thought.