Engineering in Miniature (EiM) is your friend for this:
Doug Hewson wrote a long-running highly detailed constructional series covering a variety of wagons in 5″ gauge in this publication.
Standard 12 ton mineral wagon drawing to 1923 RCH regs.: EiM Vol 8, No.12, May 1987, page 360.
BR steel 16 ton standard mineral wagon: series commences EiM Vol 10, No.12, May 1989, page 366.
Here is a searchable index for EiM.
Model Engineer has had some wagon construction plans and explanations, but nothing as detailed as the Doug Hewson series in EiM.
Here is a searchable index for Model Engineer
Regards,
Phil.