I like your design, and I completely understand the root problems (size). I'm wondering if you know what the temp of the nutrient solution is?

the nute solution is warm, real warm. 90+ right now, the temps have been high here for a few days with the heat index around 105. Surprisingly the plants are doing fine even with the high nute temps.

That is what I was afraid of with the nutrient reservoir being in the direct sunlight. I had a problem last summer with the nutrient temp being in that range for prolonged periods. I won't make that mistake again. If I cant keep the nutrient temps under control, I simply wont waist the nutrients on the plants until I can. I can give you a list of problems, but sufficient to say I learned my lesson. I don't mean to scare you, and wish you better luck. Just be on the lookout for solutions to the problem.

P.S. Are the roots turning black/brown yet?

I've often wondered about water temperature and NFT, I mean quite a fair bit of the roots are suspended in air and out some are also above the water line.

Anyway I would still build a shade structure for the reservoir , wrap it up in polystyrene and throw in a few large frozen bottles of water daily.