Make your own Regency Gown.Pattern and Instructions. For the skirt I think I'd wear a circle skirt (I think this is just the instructions, but they're detailed enough to help with any pattern of this general type!
Fabric Train idea for Mary's dress. Indian Muslin-“Dress of sheer white India muslin. The dress has a short train and is embroidered with gold threads of various weights in a running vine-like pattern with occasional single flowers.