Return air vent question

Ill go ahead and state i dont know much about home hvac

Im in the process of a big remodel on a older home. And I believe its called the cold air return vent. Its a 20x25 vent in the hallway. I am wanting to do a couple of things to it. First off i was going to go with a wooden cover to update it (http://www.woodreturnairgrills.com/) cheapest place i found them about 100 bucks for a 20x25 if anyone is looking also.


But anyways i can tell they did a horrible job they use to have a really small one (crappy patch up job too and then moved the 20x25 higher on the wall) and its vented just into a enclosed wall about 4'x2'. The sheetmetal comes to the subfloor then its just open space to the filter. I pulled the next wall wood panel down to drywall it and i prob shop vac 4lbs of dust/dirt from that area. So i would like to put a sheetmetal box from the floor to the vent to keep it enclosed.

Whats the best way to do this what size sheetmetal, hardware to use rivet/screws or even weld, kind of caulk. I want to do this the right way.

Also since they changed the size of it how do i know thats the right size is bigger always better? I would like to do a slimmer one but i dont want to cut air flow.