Unfired Clay
When making ceramics, you first form your object out of clay, wait for it to dry for about a week (give or take a few days depending on humidity), then you bake it in a kiln for anywhere from several hours to several days to make the clay melt together into the glass like substance we generally call ceramics.
While ceramics have been brought up already, greenware (or fully dried but unfired ceramics) are both more brittle and much easier to mass produce. The final step of making ceramics uses up a lot of fuel. Even a small kiln only meant for a single jug or pot can take up about 2 cubic meters of fire wood to properly fire, plus they require constant vigilance during the firing process... but since you want brittle objects anyway, you can skip this part.
An unskilled laborer could manage the entire production process from gathering local resources to a finished product well enough to make dozens, if not hundreds of these things a day. The way you would make them is to start off by clearing some dry ground, preferably somewhere sandy or loamy. Then you mix your clay which is just the right kind of dirt plus water, or better yet, you use the naturally ready to use clays like you often find in riverbanks. You then lay a sheet of clay and flatten it like cookie dough. Then using a fired ceramic mold designed by the wizard in the shape of your rune, you stamp a repeating pattern into the sheet (also like cookie dough), then using a knife or even a mildly sharpened stick, you finish off any cuts that were not fully made by the "cookie cutter". The you just leave them there on the ground to dry in the sun. After about 1-2 days, the tablets will be dry enough to handle. Since you caste them on sand, you don't have to worry about them sticking to the surface, they will just lift up with the sand they stuck to no problem. At this point you just bring them inside where they can finish drying and be stored, and you can use your clearing again to lay down the next batch.
Over the next few days, some of them will crack while they finish drying; so, you want to wait the full 7 days before casting any enchantments on them or else they might spontaneously explode.
Why this is better than other solutions
It is way cheaper; so, you can make a lot more of them. Not only are you saving a lot of time and fuel not firing it, but you are also simplifying the shape. Because greenware breaks more easily that fired clay, you don't need to make it hallow or really thin to make sure it will shatter on impact. Even eggs are more expensive because of the time, food, and land space required to raise chickens.
Snow is probably the only cheaper material listed so far, but snow crumbles in your hand too easily, and even a little bit of seasonal warming will likely cause one of your snowballs in storage to come apparat and blow up your whole cache.
The big downside is that care must be taken to keep them dry, if your weapon cache floods, you will have a big problem, but the same is also true of other historical weapons like bows or iron weapons; so, maintaining a dry armory is already a priority and available skill set at the time.
Also, you can supplement this idea with land-mines!
As the enemy army marches, the dirt under their feet naturally breaks apart as it is flattened. So, as the enemy army gets close, your wizard can go out to the main road with a bucket of water, wet the dirt if need be, and then stamp the rune into it. He then casts his enchantment directly on the ground and covers it wit dead leaves or what not so when an enemy solider steps in the mud, it breaks the rune.
