I made some enquiries through the forum here about how much wood it was safe to cut from a beam without greatly affecting its stiffness and was advised to look at the way in which aero-modellers save weight on wing formers. But even in aero-modelling sources, there wasn’t anything much about the maths of the calculation – or at least, nothing that I could follow! It seems that the aero-modellers use a fair bit of “rule of thumb”, but they cut away really quite scary amounts of timber. I eventually compromised and have cut away a large amount of the ply, leaving ribs of at least 30mm depth and with rounded corners of about 12mm radius. The picture shows the lightened formers slotted together. This has saved over 2.5kg already. I’m pleased about the saving in mass, though a bit of me resents throwing away 1/3 of my ply!

My next task, when I get a few hours spare, is to glue the formers together permanently, mark out and cut similar holes in the baseboard and then glue the formers to the baseboard.

I wrote the above almost three months ago but haven't got around to posting until today!