Ship has ribs, overlayed with longitudinally placed lathes. These are screwed to the ribs. I’ll post pics over coming days but!
I think I have a solution to do the work correctly. First with polythene tube, aprox width between ribs. Sew a fabric cover same width. Put polybag into cover, drop behind lathes, fill with poly beads, possibly with blow side of vacuum to sort of expand bag and allow beads to fill voids. We shall see. SWOT. Quick and in theory easy. Might not work, beads not falling down to fill. Maybe someone else will try and possibly improve on scheme this saving a lot of buggering about.