MississippiMan is right. Definitely go with milliskin spandex. I used a SINGLE layer of milliskin matte white spandex and got great results!
Keep in mind that absolutely everything behind my projection surface was already black, so I wasn't worried about reflections from back there, though when I did briefly have my old screen mounted behind it, there was a clear halo to the image from it passing through the spandex, reflecting off the white screen 2 feet behind it, then hitting the rear of the spandex.
As far as AT properties go, I can't hear a difference in the sound shooting through a single layer of milliskin, even before redoing my XT32 calibration. I certainly wouldn't be concerned with the AT properties of two layers.
As far as how to do a constant image height setup, the screen is a 2.39:1 format, the projector is set up such that the width fills the screen. Much of my content is played via my HTPC, which I have set to a 2.39:1 display ratio so it fills the screen perfectly and automatically adjusts content to be constrained by the height parameters of the screen. I also use madVR filters in Kodi and JRiver to automatically crop out hard coded black bars from content. As far as Blu-Rays go, anything that is 2.39:1 displays perfectly right off the bat, for anything that doesn't, my projector supports enough negative digital zoom to fit 16:9 content to the screen, plus I could get up and adjust the lens manually if I wanted to, but the priority in my setup is definitely 2.39:1 content, so if I need to accept slightly lesser quality for 16:9 content by digitally zooming out, I'm usually okay with that. In conjunction, I've found that most 16:9 content still looks very good when cropped to 2.39:1. I currently have black felt above the screen, but I'm looking into affordable velvet options to absorb overscan. this way I can just let the 16:9 content shoot onto the screen and let the top and bottom information be absorbed by the velvet. It remains to be seen how practical that is.
I definitely considered an Anamorphic lens setup, but the cost of the commercial products were exorbitant and the amount of work required to DIY one was enough to keep me happy with what I've got. I think this setup should serve me well till I can afford a 4k projector, then maybe I'll consider reworking things.
There is zero moire or screendoor effect with milliskin, it has a super close weave. The image looks great even close up and the colors pop super well, even with my fairly entry level projector (Optoma HD141x). I couldn't be happier with the results, and I sit close, this screen is 124" diagonal and we sit 8' away. The image still looks really good and the fabric introduces no issues, the images on this AT screen are better than they were on my motorized drop down blackout cloth screen.
Also, I was worried that stretching the spandex to get it even would be difficult. It was extremely easy. it wasn't critical that everything be exactly the same. Just stretch it till the wrinkles are gone and staple it down. Good to go!