Look at Lexan not fiberglass or plexi both of those will discolor and get brittle from the heat . Does i it need to be totally water proof ? I ask becuase often tho clear covers hold the heat in the space below. I love the pergolas with the canvas that can be opened or closed . This is one idea

I like the retractable canvas suggestion. Unless your pergola is structuraly designed for a roof, I wouldn’t install Lexan (or anything) on top. Wind could severly damage the structure