from my limited quick testing with a uv led, I found that a orange/burnt orange colour blocked the led's light best (was testing the celophane wrapping materials) but to block the led's light it took several layers of film to actually block the uv light so that a flouresing material did not flourese when the uv led was shined through the film at the object, so assuming you want a rigid window material, my suggestion is an inside acrylic, 10(many atleast) layers of the orange celophane, then another layer of acrylic, this has the advantage of being fairly easy to obtain anywhere and also able to be modified easily with differing strenghts of uv block(more or less layers of celophane) also the multi layer celophane does not have a huge effect on visibility and most of the effect is from the surfaces of the celophane