I have used paintable seam sealer to good effect on several cars/caravans that I have owned. Clean and dry affected area, brush it well in, then allow to dry. Test. add more if needed or paint over with finishing paint as and when required. Done once and never had to do again in the life of the vehicle. It is flexible and sticks like sticky stuff should!
Available from most car paint suppliers and the bay of E.
My boot leaked in via the lower clips that supported the rear screen (all had to be reset in sealer). For belt and braces, I applied a semi circle of silicone sealer above the position of the clip to the screen directly, to divert any liquid from the screen, to either side of the clip, also eliminating that route. The rubber screen surround covered the sealer, (ie was higher than the position of the silicone) so it couldn't be seen when back in place. Any water, then ran around the edge of the boot lid and drained away underneath as it was supposed to, rather than entering the boot void.
Lowering the rear seat backs makes it less claustrophobic for the "observer" in the boot (but maybe not as much fun!!! Especially "releasing" the prisoner at the side of the road!!!).