Having built one very similar to this, I was never able to get much below 10-12 degrees below ambient. While this was fine for ales, it couldn't get me to lager temps or more importantly cold-crashing temps. Additionally, I was always defrosting it because of the frost build-up.
I ended up scoring a free stand-up freezer that I put a temp control on. I keep it at about 40 degrees most of the time. Inside of it, is about 5 gal of water and 2 pond pumps. A control system toggles the pumps on and off to keep the fermenters at a set-point. I can crank the freezer down to 32 degrees and get my fermenters down to 38 or so, which is plenty cool for me.
This is what it turned out like:
I have since upgraded the coils to be much longer to help things out a bit.
With regard to your drawer-shelf idea, Just make sure it doesn't tip the system over when the carboy is filled and the other shelf is empty!