is there a top cover for Triton 420 (Protec ? )

Automotive Leisure Covers from £49 !!!

I think you are looking at a generic small roof cover, Maypole make them. I have the smallest one for my Familia which is up to 14'.
There are a few pros and cons which I experienced to consider, the draw back is that they cover the gutters so you get black streaks all over the van rather than just down the sides from the four holes in each corner.
Having said that it is far easier to clean the painted aluminium sides, rather than trying to cut back and polish a chalky gel coat on the fiberglass roof which is very susceptible to UV damage. You can also get slight scratching on the Windows if the bungee straps are touching, grit gets trapped between the bungee and the window and then rubs with the wind. I had one swirl which I easily got out with some Brasso wadding.

Or, you could cut the roof off of an old cover when you replace it ? Luckily I have a spsare one I can do this with and use in the summer. :)