Yeah the $5 was from a med patient friend, he wasn't looking for money just trying to keep his grow a bit cheaper.

The walls around it will keep the temps up definitely, but it also has the added benefit of reflecting the light on the sides and lower parts of the plants. Home depot has duct fans for like $30 in the ventilation section or Radio Shack has 110v fans for about the same. Either would be a good way to vent the hot air trapped inside if you do box it in. One thing you can do is set up a half wall that goes up, up to the lights so that the light is still reflected but the top is still open so temps shouldn't be more than a few degrees higher.

I suggest you lower the lights a bit, looks like the seedlings are stretching. I keep my fluoros 1-3" away from the tops and that keeps them very compact.