I've found Black Gold soil just about everywhere from walmarts in texas, to home depots in california. it's "cheap" soil because they don't add fertilizers to most of their mixes, but it is very good to grow in. It's in a black bag with a large picture of a rose on the front of it. The coco mix is in a different bag, but where you find the general black gold soil you'll find the coco if the store carries it. If you can find some perlite add some of that to it as well, but if you can't no biggie. It does have some, but not enough in my opinion.
The reason we're telling you not to use soils with added nutrients is because often times they are the wrong ones, and there's to much of them for seedlings. I tried miracle grow on my first attempt, and my babies just burned up bad, I ended up scrapping that grow. Not everyone gets that expierence, but I'm glad I did, because now I know better.
As far as a fire hazard goes, you shouldn't worry about that with CFLs, if you were trying to use a HPS or MH light, then yeah, I would be concerned, but CFLs run cool enough that you could put them in that box without worry, even if you do keep it covered with stuff.
And about the blanket and things that you throw over it, make sure your temps inside aren't getting too hot. I understand the need to hide your equipment, but you might want to look for a better solution.
As for ventalation DON'T USE CARDBOARD TUBES! They leak air! The reason you want ventilation is because the plants like fresh a little bit of a breeze. The swaying action will help strengthen the stems. Also if you control the air in the box, you control the odors. here's a couple links that should help you out with your vents:
There is a guide in here on how to wire PC fans. This is a very simple carbon scrubber. You can do a search and find more ways to make them. If all the air from the box is pushed out thru this (exhaust), the activated carbon will "scrub" the smell out.
Also look into building a light trap. You won't have to worry about it on the exhaust end if you use the scrubber, but the intake end you will. There's multiple ways of doing this, you could use 2 connected 90 degree PVC elbows painted black on the inside turned oposite ways so the light has to bend around corners, or you could cover the intake hole with a layer or 2 of cut carbon filter (like they use in air purifiers). just don't get the HEPA filters, get the carbon filters.
Also make sure your exhaust is stronger than your intake, you want air getting sucked INTO any cracks and leaks in the box. If air is blowing out thru a leak, then it's gonna smell because it's not going thru the carbon scrubber.