Created a jig from a block of hardwood cut with parallel sides 88mm apart then sanded down 5 degrees on each side. Rounded bottom corners to match inside profile of spar. Sanded recesses on both sides 20mm long and about 1.5mm deep.
Placed jig in end of spar, backed with another piece of wood then clamped in vice. Tapped flanges into recesses with rawhide mallet to form joggles which allow the end rib flanges to sit flush with the spar flanges.
