Perhaps ONA gel might do the trick for just one plant. That and a small, plug in, air purifier. Or else just buy lots of masking air fresheners. ONA gel is supposed to be good but I would always go with a good carbon filter. I have 7 plants in week 6-7 of bloom, my extractor pulls the air in the tent through a large CAN9000 carbon filter (scrubber) and I cant smell anything.

I bought a cheap carbon filter on eBay but I had to get rid of it before I even flowered. You CAN'T beat a good carbon filter. Not a chance.

Getting back to your grow, how do you expect to grow a plant properly in a PC case ? Even get a 50cm x 50cm x 75cm (high) cardboard box and line it inside to be a grow box. Nobody would suspect a cardboard box. There are many ways to create an inconspicuous grow box. You will have a hard time growing in that little PC case. But I'm interested to see how it goes. It must be like a little bonsai tree

Oh, you can check out my LED grow journal by following this link: LED Logger

There is an update which talks about the importance of a good carbon filter. I also bought negative ion generators (€30 on eBay) to control the smell when I used the old carbon filter.
I don't have them turned on any more since I got the CAN9000.

Good luck and take care