cavity closer

I've been building my extension walls (break now for bad weather !), and the only opening I've started to form at present is the start of the french doors (more properly, french windows !). I assumed I was going to fit cavity closers after the walls were built - the type with say polystyrene core and 1mm thick flanges, so no real affect on the brickwork opening required. However, I have been made aware of the following :-

The diagrams of closers I have looked at have clips to build into the wall as you go. Is this required, or is fitting at the end OK ? New builds under construction around the place all seem to have the 2 leaves and insulation showing around the openings, so I assumed fitting after is OK.
I've seen more sturdy PVCu closers that can be clipped together with corner sections to produce window profiles to build around. Anyone used these, and are they worth it ? Thanks, Simon.

You can get ones that just go in after. I have used the Jewson version which just consists of a plastic plank with a flange on the back. Against this is a foam insulating strip with several lines scored along it. You simply cut along one of the lines to trim off the foam to the correct thickness, then place the closer into the cavity, and nail or glue into place.

Quite expensive for what they are though at about £5/m

Not tried them... I would expect much depends on what you are planning on installing in the opening. The modern trend of placing windows and doors close to the outside tends to mean there is no closer exposed to the outside, and the inside gets covered in plasterboard etc for the window/door reveals.