OK. What he's done is what I've done with my CC. If you have a look at your owner's manual, you'll see that the Megane CC doesn't officially come with a spare wheel, but with one of those useless tyre-gunk repair kits. The thinking behind this was to maximise the boot space when the roof is down by not having the well cluttered up with a spare wheel, thus freeing up extra space for, say, a small suitcase or bags etc.
So looking at your pic, on the left is some homemade ply with a cover, which you should use if you decide to keep the spare wheel. Do this, and you will have to store the other Renault insert in the garage or loft, as it now will not go in the boot, because you've just covered the spare wheel well with the home brewed cover.
To the right is the boot insert that comes with all Meg CC's. But you will have to lose the spare and revert to the repair kit, which fits into a circular polystyrene insert that goes underneath the boot insert, right at the bottom of the well. If you choose this option however, as mentioned you gain a little extra boot space when the roof is down. So basically its one or t'other.
FWIW, I'd keep the spare wheel and sacrifice the boot space, as I don't think much of those emergency kits. You're lucky that you have a spare that came with the car. Most including myself who choose to have a spare wheel, get a cheap one from the local scrappy / breakers. Make sure that you have a wheel brace and jack!
Hope that all makes sense, the choice is yours!
