Earlier iron patchboxes were not that much different from their brass counterparts, and you could have 2 piece (e.g. Bogle) w/o sideplates, 4 piece (with sideplates), captured lid, or one like Dr. Tim shows. If you do 4 piece or captured lid in iron, my preference is to keep shape simple (maybe following the "geometric" types), little or better no engraving, possibly a little file-work, although there are a few examples of iron patchboxes with more engraving and even silver inlays.