I'm trying to hide my sub in a cabinet, one one side of my stage.
It is a front firing / rear port 12" sub. i'm planning on building a cabinet and leaving the front open, opening would have fabric on it. I'm wondering what
to do about the top or the one side? should i leave that open as well?

ALDOG - I think you might have better luck with your question in the DIY Speaker forum; they have some really knowledgeable folk over there ;) My first impression would be that a front firing, rear ported box would not work well in a cabinet. You can certainly try it and listen as every case if different but my guess is it will be far from ideal.