To sort of begin at the beginning:
Extruded plex warps easily.
Cell cast does not.
Thickness doesn't really matter except fir rigidity
Most say twin wall polycarbonate is stable.
Also easy to work with.
Most plastics have a sharp cuttoff in transmission below 400nm. Not normally an issue in fw lights.
All lose some light though most are in the 10-15% range. Even "dirty".
Another option us eggcrate plastic.
You need to define your needs.
Lower evaporation?
Structure strength?
Fish jumping proof (what size fish).
As mentioned there is also netting material.
Size and color matters.
Globally most materials have been quantified to their light characteristics.
Worst common cover for light transmission would be fine mesh black screen. Which us sort of obvious.
Thickness does matter a little.
Polycarbonate absorbs more visible light than plex.