Question about corners of a small booth

Hello to all, I'm currently building a small rectangular vocal/guitar booth (2mx1m50) made of wood and I'm thinking of the corners. I'm thinking of putting one small board of wood "across" the corner : the upper part would be used as a silencer for the ventilation and the lower part would be filled with rockwool. I'm not sure this would be a good idea because I don't know of this "box" would react to sounds. Should I put holes in the lower part of the board or replace it with fabric ? How could it become a kind of corner trap ? Thanks for your advices ! Here is the plan (the board is painted with two different colors for clarity) :

depending on what you're trying to do with the trap. i'd just make it without the wood front, and if you need some high end preserved, put a layer of plastic under the cloth.

Thanks for your advice, that's one of the options I prefer. EDIT : maybe a stupid question, but what kind of plastic should I use ?