Hi,

most RV places sell an aluminium extrusion, in 10' lengths. like a C with a
flange on the opposite side from opening. Flange gets screwed to rain
gutter, I've seen it on underside of a Westy gutter. Could also go onto
roof, aligned with the lid seal when closed.  A tarp with a fat hem is
used, slide some 1/4" rope into the hem, the slide the tarp/hem/rope along
and into the C opening. Rope keeps the tarp from pulling through the C gap.
File the corners of the C at each end to avoid tearing the tarp.  Still
need the two tent poles+lines to hold it up away from the van.  The C rail
is $1.25/ft. around here, black or clear finish. Yeah, I know you can't
roll it out half way on a windy, It also costs about $25, wadddya want for
that price.  Make a half width tarp maybe?

bye, Tim