Once you have made your holemouth jar, it is quite a simple matter to make a little lid for it. To do this, it would be best to save a little of the polymer clay that you used for the jar itself, so that the lid and jar will both have the same colour.
First, flatten the clay into the form of a disc, which is roughly the same size as the opening of the holemouth jar.
Then impress the mouth of the storage jar on the disc of clay, to create a reverse image on one side of the disc. Remember, the jar itself should already have been baked so that it is hard.
Now, place this disc in your oven and bake at the required temperature, but only for about 5 minutes. This will harden the clay a little, but not fully bake it.
Take the disc out of the oven and allow to cool. Then flip it over with a blade so that the reverse image of the mouth of the jar is below.
A handle for the lid can be added at this stage, if desired, with a small ball of polymer clay.
Finally, just replace the lid into the oven and bake according to the instructions given by the manufacturer of the polymer clay. When finished, the lid should fit neatly onto the holemouth jar, even if this was a little irregular to begin with.