The feather-edge boards cladding the frame (right) need covering and the window needs something to butt against so I nailed 38×19 roofing battens around the frame like this (below).
Then applied a bead of silicon sealant and eased (OK, levered and hammered) the windows into place. These windows came from the back of a house that was being extended and are only a few years old. There are two windows positioned so I get a good view out but they are almost impossible to look in.
The french windows came from a friend. They had been used internally between two rooms, but are equally suitable for external use.
They will let in plenty of light, but are not very secure, unless you do as the french do and fit a shutter. A 3′ ledge & brace door is ideal, but I couldn’t find one, so this is home made and extremely solid. It’s fitted with a Yale lock so it should be safe enough.