I like the use of that polycarb greenhouse/storm window material as lids. That's what I'm going to use on my 4x8 tank as well. It's nice and light, and mind bogglingly tough. I'm not sure about your pipe hold-down method. I am sure it will work, and it's quite clever, I just think it would annoy me after a while having to slide the pipe out to open the lids.
I always wanted to work out a way to use rare earth magnets to hold down lids on a big tank. No latches to mess with or anything. Only real issue is retaining the magnets in both the tank and the lids, and protect the magnets from corrosion... but I digress!
Tank looks great overall, looks like a LOT of sanding!