David,
If you are member of the LDMCC, you could try contacting Dave Lawrence - he may be able to help out with an original drawing of the toolbox and lid as fitted to the S6 model (I believe the same toolbox was used right up to 1938). Some years ago I fabricated one of these toolboxes from 1/8" alloy - it's basically a bit of folding and welding - there is not a lot of forming to do.
Regards,
Eddie.