I made some of these masks yesterday and also watched I Am Legend as my first scope movie.
I was unable to find the product you mentioned at Lowe's, well I wasn't able to find that product number. I took the product number to the customer service desk and it wasn't in their system at all. In their "styrofoam" section there weren't any 2x8 sheets. I have a 110" 16:9 screen and actually went with a 4x9' [yes I did type nine] because I wanted to trim it for an exact snug fit. I purchased the tougher styrofoam. I think it was called "R3" or something to that effect. I paid ~$13 for the sheet, even though I didn't need most of it.
I used spray adhesive to glue the velvet to the styrofoam panels and it worked perfectly. The adhesive did have a very insignificant reaction to the outer layer of the styrofoam that made it seem like it melted [very slight reaction]. I found this to be a good thing as it allowed me to drag my finger across the edges of the panels and actually make them more straight.
I'm glad I made these panels. It's a worthwhile project that will cost you very little and only take about 2 hours of time at most. I have them in place in just a few seconds. This will last until I attempt to put my engineering degree to use and develop an automated masking system.
Thanks for the tip!