Hi,
A refugium does not have to be large, the critters you want to cultivate does not take up much room...even a 5g tank makes a good refugium. I used a little in tank refugium from CPR, and it worked great. They also have HOB refugiums...all under a gallon and all worked awesome.
A sump system with a refugium chamber is ideal, but not necessary. IF you have a drilled tank, I would absolutely go with a chamber dedicated for a refugium...if you don't have a sump system for filtration, the above suggestions are just fine.