Depends on the material. Several choices. Also depends how deep the window is mounted. I've got some bullet proof glass I picked up off CraigsList I'm going to use someday as a pond window and just hope for the best.
I've read threads of people talking about the scratching issue and also that the plastics may not be as clear as glass. My bullet proof glass panels are very scratched up and not that clear. I believe it's polycarbonate.
I have read on here about people using plexiglas and scratches problems
So I'm going to use glass, after a phone call to a local glass firm they advice me to use
Triple layer toughen glass to the thinknes of 1 inch and for a cost of £308+vat :-0
I will shop around too see if I can get it cheaper
I be starting the build next year after the frozst so gives me time to get everything ready for the build