Start by getting a VERY old pc.
They are MASSIVE.
Buy new fans, you'll need at least three, one for pulling air in, one for pulling air out and one to blow on the plants.
You could probably get away with Two if you position the Intake fan to blow the new air over your plants.
You'll have a difficult time making your box airtight too.
I'd suggest Using something thin like ply board to make a box inside the PC case and painting it White.
I wouldn't suggest using Cardboard because, altho many here say it won't burn there is the chance that it could..
So you have your pc case, gutted it out, make a "box" inside it so it's airtight.
Making the box won't mean you lose any room, just make sure you get some thin wood!
Odour control may be an issue so check out my thread on DIY carbon filters.
https://www.rollitup.org/indoor-growing/90893-ultimate-odour-control-thread.html
You may want to use a plug in air freshner in conjunction with a carbon filter.
you could probably get away with LST'ing two plants.. make the Soil tubs low but wide, as this will stunt it's growth making it less likely to over grow your pc case.
Remember, your plant can double, sometimes triple in size when you put it into flowering.
I saw a thread on another site about a PC grow that was great, if i find it again i'll post the link on here.
Go with CFLs for this, you could probably fit about 4/5 with two either side and one pointing down in the middle..
Hope this helps, don't forget to +rep too