I had a very detailed reply which somehow disappeared so I will give you the short version because I couldn't possibly duplicate such a masterpiece. :P
The diffuser/mesh is for the inside of the egg tub. The 1/2" screen is sold as "hardware cloth" and goes over the zig zag shelves that came with the cooler.
You could fit three strips of 3" flexwatt but I doubt four. I kept mine on the back wall only. 11" doesn't put out enough heat, IMHO. You may need two fans at the top.
Putting a removable shelf at the bottom, covered in hardware cloth, would work fine. The glass felt like it was glued in so you may have to break it. I used small water bottles at the very bottom so I would have room for another fan at the front blowing up to circulate the air. I would not rely on the water bottles to support the bottom shelf. They may fall out along with the shelf and egg tubs.
I removed the coolant panel and everything else except the light. I rewired it so it would still work. Everything under the box was ripped out. Cut the lines outside in case it leaks.