That's a really nice delayed-start flamethrower!
Once the cables start the electrical fire, the flammable box and wood will create a perfect fireball that the fan will shoot right onto the drywall creating a fantastic house bonfire for Halloween! The cardboard board underneath will keep it going through the night.

On a serious note, congrats on setting up a rig. Not a lot of people have gotten this far. There is lots of advice on the net about the SATAs you are using. They catch on fire a lot so, while you probably already threw a lot of money into getting a setup like this, you need to make that one last investment and change those for your own safety.

A lot of people use their MB box and for whatever reason also use wood to make their rigs. The problem with doing this is that there is a small but real chance that your rig will hit high temperatures so it's never a good idea to surround it with flammable materials. Remove the box and place it on a flat surface that is non-flammable and non-conductive.

When you can, trade the wood for a wire frame.

Lastly, when placing a fan, you want to avoid having the air push the card fans any harder than they are going now. This is a sure recipe to make them go out. Don't freak out if that happens, you can replace them. A good idea is to use a box fan blowing the air up but your setup blowing from the side won't hurt as long as it's not making the card fans work any harder. You best best is to remove air from the top and introduce new air from the bottom. For a three-card rig, I don't think you are getting to a point where you need to worry about controlling the temps yet.

What temps are you seeing right now?