I never try to gel my soaps so I am a little lost here. What do you mean by "empty container" and what do you use it for? Thanks!
Animara provided you with an example of the type of container that you can use, but I'm not sure if she saw your question about what it is used for. I don't mean to speak for her, but I thought I'd clarify that part of the question. When the saponification process is going on, it produces heat. If your soap is sitting out somewhere uncovered, that heat will dissipate, and depending on a variety of factors, like recipe, mold shape and size, the temp of the room, the soap may not gel, or partially gel, or even gel all the way. After you've been soaping for a while, you will probably develop a preference for gelled or ungelled soap. There are ways to insure, or at least encourage both, because most people dislike a partial gel, which produces a soap that gels in the center, but not to the edges, so that the color is not uniform throughout.
For those that like ungelled soap, putting the soap in the freezer is an option. Although soap can be stubborn and gel anyway, but it is much less likely if it's in the freezer. For those of us who prefer gelled soap, we can CPOP, putting the soap in a prewarmed oven, then turning the oven off and leaving the soap overnight. Or as Animara suggested, you can put the soap into a container of some sort, with a top that closes to hold in the heat. You can add a heating pad to add a little extra warmth, or cover it with a blanket or towel, creating a soap "incubator". I always CPOP, but sometimes I use Crystal Lite containers as molds, and I'm not sure what would happen to them in the oven, so I have a small styrofoam cooler that you can buy for a few dollars, that I use for those molds. What we're trying to do is to hold in enough heat to encourage the soap to gel all the way to the edges. Hope that answers your question.