Masked window

"The next step was to dry fit the frames and mask them on both the outer (fibreglass) side and the inner (Plexiglas) side."

(Masking like this looks fussy, but saves a lot of clean-up time. The simplest way to do it is to dry-fit the frame and mark the position of its edges lightly on the window and the fibreglass. Then remove the frame and lay down tape to overlap this line. Dry-fit the frame again then run a sharp knife around the frame to cut the tape. Remove the frame, peel away the tape that was under the frame and you have a perfect mask without a lot of fiddling with little bits of tape.)