From the dimensions given above the M7 x 80 bolts used on Citroen 2CVs to fix the lamp bracket to chassis might be suitable for adaption. Available from 2CV specialists such as Der Franzose at around £2 each.
Jack 1985 4/4 T/C 4 seater Royal Ivory 1934 F Type 2018 Pembleton SuperSport (self built).