Fermentation Box Questions

[Nimbus3000]

New member of this forum, think I finally found the correct place to post this (my apologies-not real familiar with this). Just getting started into home brewing and currently trying to get all of the basic equipment before I start on my first batch. I am currently working on building a fermentation box based on the Son-Of_Fermentator design as modified on the Wort-O-Matic web page. I have attached a drawing showing the plan views of the ice chambers and fermentation chambers. I did not include a cross section but the plan is to have 1 1/2" foil backed insulation on both the top and bottom of each ice chamber and 1 1/2" insulation under the bottom of the box. The bottom of the fermentation chamber will have 2" of rigid insulation and the top will have 1" foil backed insulation. The inner and outer boxes will be built with 1/2" OSB. There will be 2" insulation on the top, back and sides which will leave a 1/2" gap to be filled with pour in expanding foam (same stuff that is used in boats for flotation). The front will be filled with pour in expanding foam against the doors which will be treated with a release agent (this should provide a tight fit as the faom will shape to the doors). Cool air will be pulled from the two outside chambers into the middle chamber, up through a 4" duct and into the top of the fermentation chamber. This will be controlled by two STC-1000 controllers. The ice chambers can hold 3 24 pound blocks of ice in each chamber (these are the large plastic jugs that kitty litter comes in). 90% of all the materials used are from left overs from other construction projects (which is why I am looking at this instead of a converted freezer (plus when it is done it will be finished out to look like an old style ice box). My question to the more experienced folks out there is whether I have a major design flaw here that would make this unusable? It seems that there are a lot of posts on this site concerning fermentation boxes so thought that this would be a good place to ask questions. Thanks for any feedback.