Michael's sketch with dimensions for his board, used as a starting point.
Trial pieces in hardboard, originally used for a board then modified for the bar, sill side on the left, tunnel side on the right.
3/16" aluminium offcuts, with the wood plugs to go inside the tube.
General view, the side at the top goes against the tunnel.
Right side against the tunnel, angled in at the front.
Installed, just snugged down where it takes up a natural position along the carpet edges.