If you use a plastic pen cap, or section of plastic rod or straw for a miniature gazing ball stand, you will need to close the base and give it a wide area to balance on in your miniature garden. If you have sanded any curves flat on the top of your pen cap, you can glue the top of the cap to a button, flat bead, or flat charm (I used a plastic snowflake) to help keep your gazing ball from tipping over. You could also glue a piece of wire or wooden dowel into the tube if you want to secure it by pushing the wire down into the soil in a garden or garden pot.
If your gazing ball will be set outdoors or in a pot with living plants, make sure your glue will stand up to moisture. Plastic cements, hot melt glues and silicon glues will usually work to glue the base to the pen cap.