Jigsaw

My windows were from Lewmar - yacht windows guaranteed to about 50m below sea level! And openable! Nice.

Fitting instructions here...

Great things, but expensive I guess, mine came with the van!

Notice the masking tape? Stops you scraping or splintering the fibre glass when cutting. You need a good, sharp metal (fine tooth) blade and high speed jigsaw, the cordless one was less good in the end as the battery was so heavy so we switched to more powerful corded one.