You could use a 30 gal long with some egg crate. You should be using water with as close to the params as your display tank. I've place my frags in my display tank and or in my refugium. Being that both have excellent lighting and it's the same water chemistry as were they're normally live in.
besides I don't have any extra room for a frag tank.
If I did it would go towards and even larger tank. my 29 and my 55 are nice but I want to have a 200+ gal tank.