In terms of acrylic don't dig too far into it. Use standard clear 1/8" cast acrylic sheet, it will not warp and will not remove any visible light worth mentioning (0.5% of visible light is removed per inch of thickness). There are difference classes of acrylic for removing UV light but LEDs don't emit UV and you can avoid it to be safe by not purchasing a special type of acrylic. Also optics are acrylic....
http://www.sdplastics.com/acryliteli...FUVtransTB.pdf
And most Rona's carry the 1/8" cast clear for a good price.