you need a way for the water to reach the refugium. if the tank is not drilled, you'll have to purchase a HOB overflow box. also, you will need the actual refugium.
i'd suggest partitioning a basic glass tank(the largest one you can fit in the area you have allotted) in 3, and then have the refugium in that. there are many designs for sumps with refugiums, and i'm sure you can do a search here and find plenty of info.
i like the- intake, refugium, return layout.
or, you can simply purchase a sump built with a refugium in mind, but these are very expensive.
what to put in it is debatable also. what will your refugium do? just filter the water, or will it be a place for pods to grow as well?
if just filtration, you can simply toss some macroalgae in it and put a light over top and you are done. if it is to be a place for pods to grow as well, then i'd suggest a sand bed and maybe even some rock/rock rubble at the bottom, along with the macroalgae.
you can even use this as a place to house a deep sand bed. (4 inches or more)