Here’s a mezuzah tikkun I like. It’s from a fourteenth-century French book: a copy of Mahzor Vitry, mostly prayers, but also some halakha and miscellania in the back. The link is to the digitisation provided by the Bibioteca Palatina of Parma. This is one of the pieces of miscellania in the volume.
You’ll see he has the same word-wrapping we have today, but he has some modified letters lurking in there, and he also has invocations, angels’ names, and mystical diagrams in the margin.
It appears to be written against the lines, perpendicular to the text on the reverse. That’s because the other side of the page is a tikkun for tefillin, and the tefillin tikkun, being long and thin, is written landscape on the page to fit it in.
We know from similar mezuzot in the Cairo Geniza that the way you do a mezuzah like this is you have a huge left margin, and you put all the angels and things in there on the line indicated. I don’t remember off the top of my head if the extra bits at the bottom go in the bottom margin or on the reverse side, but if anyone has the urge to write a mezuzah like this, you can look it up. The next page in the manuscript has instructions about mezuzah-writing, in particular which days of the week and times of day you should write them (those instructions did not become mainstream halakha).
Rambam says this kind of mezuzah is absolutely 100% assur. He does not want you using the mezuzah like an amulet, to invoke guardian angels. For him, the mezuzah is strictly a mitzvah, not a superstition. However, Rambam did not hold full sway in this part of Ashkenaz at this time, which is why we have this tikkun. Rambam also says that it’s assur to re-use dirty tableware without washing it first, if I recall correctly (Mishneh Torah/Kedusha/Forbidden Foods/17:30), but in any case you should proceed with caution.
