i used 1/8" plexi on mine, which i beleive comes out to just over 3mm and it works just fine. i'd be a little concerned with the thinner stuff being too flexible..but it really depends on how you are doing it. if the Tmolding covers the edge of the plexi, then it should be fine. if the plexi sits above the t-molding, i'd be afraid of it getting caught on clothing and getting pulled up.