Depends how much you want to spend. Spandex is cheap and a lot of people use that - usually two layers of white, or a layer of white over a layer of black. Not all types of white Spandex work well so you have to find what others are using that doesn't have any visible issues. Jagdeep sells Filmex which works with no issues but is more expensive, but you don't have to worry about the results.
Other materials like Seymour XD or UF are more expensive but work well.
There are a lot of threads about AT screens so try the search and see what others have used.