Having used phase change and various home-made sub-ambient set-ups in the past, I have gone through the horrors of imperfect insulation often enough that I don't wish to go that route. I rarely keep the same motherboard for much longer than six months as I am sure is the case for many for most of the people here. The only fail-safe way of insulating is IMO complete encapsulation using silicone, supplemented by heating the bottom of the board. God help me if I want to change the processor
What attracted me to your design is the lack of the need for any insulation. In the past, I used a tec waterblock with an Oven Industries controller. Check them out, they make reasonably-priced controllers. However, your design is much more elegant and a heck of a lot simpler, not to mention less costly.