Let's talk about something other than the famous honeyjar and glass bottle spells. For container spells it is said the longer you work on it, the more powerful it becomes. Sort of like wine made to age, and the amount of focus and intent it picks up from you the spellcaster and the spirits.
The symbolism of a container spell is that it is a place of ideals created by the conjurer, in which is made to manifest in the real-world. For example a healing container, filled with hyssop, rue and lavender. Putting in personal concerns and sealing the container is putting the person in a place of healing and safety. Likewise with nasty intent, with nasty ingredients (rusty nails, needles broken glass) the victim is put literally into "a world of hurt" by the conjurer.
Of artificial containers (those not found naturally in nature like the egg) I like most of all those made of metal. Tins, cans, boxes. These are light-weight and durable. Very shakable while chanting. They come in every imaginable shape and size, and cry out to be decorated. (You can even write your petition on the outside as a reminder, especially when you have many container spells.) I seem to be irrationally drawn to them for no obvious reason. They just seem charming to me, and they surely will develop scratches and dents in time as they are used.
For these sorts of containers no liquids are used within them. Just the petitions and herbs, powders. Maybe a dash of sugar if things are needed to be sweet. I like capturing the smoke from incense before sealing them.
The sort of container you pick often reflects what sort of spell you want it for. Candy tins for positive work, and chilli sauce bottles for negative work. Medicine tins can be used for healing. For cast-off evil work, go for something like cardboard. (So that you can easily torch it once you are done with it.)
Pictured here are some of my personal container spells.