Lee I'd get rid of that centre board, it's only in the way, build up the long board to the same height as your top rail, now take that centre board cut it down to about 50mm, now screw it to the built up long board. Use this to swing over the frame when wiring then swing away to remove frame. Your frame shouldn't lift up when wiring.
Check out the pic from Jim134 esp the doorstop to keep tension on while wiring.