I ordered my unit installed with the awning so can't help with an aftermarket recommendation.
I can help, maybe, in giving our impression of a slide-topper in general. We've had trailers with and without and got it with the current trailer to reduce the sap, leaves and other debris that will fall onto the slide-out.
Sap is the real issue for us. Since we camp often in forests (including pine woods) we've found that the topper does a good job in keeping the sap off. This is important because, once that stuff hardens, it can cause harm to the seal at the top of the slide. Keeping if off the slide, as opposed to cleaning it off before we close up, has been worth the cost to us.
Also, the twigs and leaves are kept to a minimum so we don't worry nearly as much about that issue. A quick check under the topper before closing is important to make sure nothing blew underneath, but so far we haven't found any.
It does its job well.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD