You can see loops of masking tape on the sound-bars and cut-off bar. I'm about the put the soundboard in, press it down on the tape and carefully lift it out so I can mark out and glue the bars in place on the soundboard. 
It occurred to me to wonder how this was done 250 years ago. Then I realised that I'd put the base-board on too soon. Without a bottom, one can mark round the bars easily enough. Sequence - that's what it's all about.