Is it the fairly standard MPK skylight? You can remove hatch by unscrewing the 4 small screws in sprung arms then take to caravan shop to match up. As pointed out, sizing up by measuring can be confusing.
Maybe I will remove it and check the measurement but I am pretty sure its 300x350, if I do it most likely will come out in kit form.
For me to make a trip to the local caravan shop would mean a 12hr trip by sea as I am in Shetland where hurricane Gertrude made her presence felt !!
Caravan damaged it is old so not the end of the world however both my classic cars have also been damaged and that's just not funny.
I had to replace my skylight after it blew off in a storm a few years ago.
As I have an older van (1969), on removing the old one, I discovered my skylight was not a standard size.
I bought a new one, as close in size as I could to the old one, but I did have to make the hole larger in order to accommodate the new skylight, which was easy enough to do with a small saw, some new batons and mastic sealer.