Inner back panel --- cut vertically down centre line & then slide/ fit each side without paying any attention to other side. When happy with result there should be enough over lap to join both sides. I bent riveted the top to the seat belt bar/tube. For the bottom I made & welded an inside/out triangle ( 120 degrees or there-abouts )
Rivet panel to this near up-right web & forward web will carry the back end of floor, which again I cut in two & bent to give the upright of the tunnel. ( allows full access to prop.
HTH we are down in sunny Cornwall now so not much chance of up country visits.