You already did all the hard stuff constructing that wooden surface.
Slap some OC703 panels on it and call it a week.
You probably need around 8 if you double stack em. You're talking 48 dollars more in materials plus taxes.
4" thick of OC703 in that little space will make a very big difference on your vocals.
Just back the mic into one of the corners and the OC703 will eat so much of the ambience it wont be funny.
Add the comforters behind you from the ceiling for a much drier sound than that, but to be honest, just some moving blankets will do you just too!
You can never have enough OC703.
Works on drum recording in a pinch too!
Peace
Illumination