lexan sounds good, seeing as how it is unbreakable, looks like the perfect cover for something
I'd go with 1/4" mainly because of that 35" length you mention ... the 1/8" stuff warps real easy
I wonder if it will yellow or fog due to exposure to the heat and radiation from the lights?
depending on the cost - it might be cheaper to go with 3/16th or 1/4" tempered glass, which would be nearly as durable, and less likely to warp than plastic, and, will not discolor from exposure to heat and radiation.
a salty would say - leave the top open, it's better for evap
planted tank, a tight fitting closed top is your best friend for keeping CO2
trapped inside the water where it belongs!