That is a nice setup. Ignore what these guys said. Yes the box will absorb some heat, but compared to the amount of air you draw in, it will do its job nicely. Good work. That box appears to be plastic, and plastic is a good enough insulator. If in doubt, get some of that mylar bubble wrap and line the inside of your box with it. I would tighten up the seal around the edges of the box a little, just to prevent air from bypassing it. You can get some split foam tubing at the hardware store that is used to protect pipes from freezing. If you put that in the edges of your box, it will form a tight seal.