in that case i think you should get the largest refugium that you can fit under your stand.
since your tank is already in place so i am guessing it's not pre-drilled with an overflow. therefore you probably need an external hang-on-the-back overflow that takes water from your main tank down to your refugium.
for the refugium itself, it's basically a sump with sections. some buy pre-made refugiums to save the trouble (me), some build their own.
there are many posts about it in the equipment section.