My fibreglass tank is about 6x4 and when I bought it I knew I needed a good strong canopy on it. I can get polycarbonate sheets from work, single and ply, but just throwing one large weighted sheet on top and calling it done didn't appeal to me one bit.
If you want to do something you have to move the whole thing, and that means the droplets of moisture on the underside would spill everywhere, nah, not for me I figured.
So I custom built a heavy duty wooden framed canopy with a large centre hole for a tri wall polycarb sheet, my light fitting sits on this, and six smaller holes around the edges for HDPE plastic panels to sit in.
The two front edge panels come out for feeding and maintenance, the rest has never moved since it was installed in 2019! I absolutely love this canopy.