Another way... [considering you are making a physical model and 1mm is optimistic!] Is to use Slicer to make some formers across the 'horn' at say 25mm c/c. Cut these from ply and build a 'former' around a central spine. Add a roughly cut over-sized sheet to one face, temporarily tacked in place and then trimmed to match the 'former' on both of its edges. Repeat on an adjoining side scribing the abutting edge. Trim the other edge of this second side to the 'former'. Repeat for all four sides gluing the sides together - avoiding attaching it to the 'former' pieces. Remove the temporary tacking and take out the former - if the shape is very convoluted make the central spine removable [wedges?] and remove the 'slices' individually...