As far as mathematical options go, the best ones I know have already by suggested. I will suggest a different option, used by the Spartans. The Scytale. Basically you wind your medium around a rod and write along the rod. When the medium is unwound you get nothing but shuffled letters. The description in Wikipedia makes this most basic implementation appear easily breakable. While I am no expert, I suppose a few twists could be added:
Don't align letters with stripes.
If you always place your letters on the stripe perfectly, you have nothing but reshuffled letters. This is easily breakable, by statistical analysis of the letter frequency. Instead use very, very thin stripes, so that mere dots are found on each, offering no clue to which letter they belong. This might be difficult to reproduce by hand, but <PERSON> will let a machine do the winding for him. 805
Very neatly and reproducibly.
Don't use a simple rod
In the event that the stripes are found, the cryptanalyst will have the patience to test all kinds of rod diameters. Therefore, don't use a rod of fixed diameter, but rather a complicated shape, similar to what you get by spinning an elaborate key around the long axis, or the ones found here.
Disguise the data as everyday objects
Unfortunately, I do not know the shape and size of 19th century sewing spools, so I will take today's situation as a guide. If I had enough trusted technicians, I would use sewing yarn, or something resembling it as a medium. I would manufacture a machine typing on the yarn, while it is of course wound around the intricately shaped rod. It would type in tiny letters, wind and unwind the yarn for me and reading would be easy with a magnifying glass. When storing the data, I would wind it on an ordinary spool. I would take care to leave some yarn blank, so the outside of the yarn (when on an ordinary spool) would be blank and inauspicious.
If my typing/printing technique is good enough, I can even hide my yarn inside Gobelins, carpets, clothes, for longer travel.
Problems
Yarn has an axis of rotation. To avoid that becoming a problem, you must have the ink seep through the yarn. That might be difficult to achieve uniformly. And you must leave blank layers, to avoid damaging layers below. 215