I think you'd have to pour the cement in a bucket if you want to be able to move it, Jane.
Yes, I made 3 gallons of concrete with cement,
in a plastic 5 gallon bucket, inserted my pre-cut 4x4 and made sure it dried, fully (2 days), and then took my sawz-all and cut the exposed top 3” of plastic bucket.
I then dug a hole and mounted my new mailbox & post sleeve on it.
Cement is one of the ingredients in concrete.
Cement+water+aggregate = Concrete.
A DIY’er would typically buy a bag of “sackrete “ (concrete pre-mixed in a bag), at a big box store for about $6. It has the aggregate (sand) and cement mixed together. Think “cake mix”.
Pour it dry, into a suitable large bucket, add water & stir like crazy.
In this case, your mixing bucket is your sign post base.
For specific needs in concrete, like structural characteristics, there are specific types of cement.
Here are some videos for you that have a lot of helpful information.
Hello Jane, here is a simple design that looks pretty sturdy