LBSC's design for his 'trailer wagon' for Tich is covered in Model Engineer magazine, 2 April 1959, Vol 120, Issue 319, pages 414 – 416 & 423.
There is a general arrangement drawing, and reduced detail dimensioned drawings of underframe, 'tank' (body), wheels, buffers, axle-boxes, etc., accompanied by a descriptive construction text.
Regards,
Phil