This is awesome, and you've inspired me to look into using my Shapeoko 3 CNC router to make some parts for my printer out of aluminum. I especially like how you have utilized the 2040 L brackets. I've been running a pretty stock FT-5 for almost a year now, the melamine parts hold up surprisingly well, but I've never liked how the brackets are on the outside of the frame. It severely limits clean enclosure opportunities. I've been looking at enclosing my printer more and more lately, not for abs or heat control per say, but more to help keep the dust out of the printer.
With all that said, you have a nice bit of documentation here showing pretty extensively what you are doing, however, would you be able to explain your y idler set up? The stock configuration has the idler sandwiched between two melamine pieces. Looking at my printer, I am having a hard time trying to see how to do it differently that would allow the use of the brackets for the frame, unless one forgoes the top plate. But then I am not sure how strong the idler would be, as the tension on the belt would create quite a moment on the small idler shafts.
Again, Great write up, and thank you for your help, and inspiration!