...a product that would get down in to the indentations better. Two products were suggested that are more sprayable. I tried Silk Shine dressing and Natural Shine dressing. These also worked very well, keeping the color looking good, and the Thule roof boxes went out looking fresh and new. I have gone through many bottles of both of these, but Silk Shine is my clear favorite because it sprays on, evens out quickly, and doesn't give a super-shiny gloss that looks fake. It does an awesome job of keeping the boxes looking brand new, protecting against road grit and damage from the sun's UV rays. 
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The only downside for me is needing to do it every time the boxes go out and come back. I use a lot of these products, and the store sells the products at full MSRP, so it gets expensive to treat the boxes every week. The store associate says I can reapply daily if I'd like--maybe he didn't understand my concern at the mounting costs of spraying a fleet of roof-mounted Tupperware on a weekly basis. A 16 oz bottle would do two boxes for a month if I was conservative and never used a clean applicator sponge. Of course, that sponge gets dirty dirty dirty. When a box comes back, I wipe it down with a clean, wet towels, scrub the front and underside a bit extra for the extra bug-bits, and then treat it with the UV protection dressing. When a customer is set to pick up a box, I give it another spray of protectant to make the box look awesome and ensure the box is protected from the elements on the road.

Any thoughts on a product to use that would last for 6+ months? It needs to protect against UV rays and road elements, especially on the front of the box where many a bug has crossed over the rainbow bridge. 

On-going maintenance continues. Detail Day has gone the way of used equipment--most of my older boxes have been transitioned out of the rental fleet and replaced with brand new boxes, so the trick is to get them protected right away when we take them out of their box and bag that they ship in. I'm wide open to suggestions. I'm also wide open to product sponsors who want to test and have their products featured. People are impressed with our standard of maintenance, so I brag about Chemical Guys.