All you need is hot glue, a spring, and a piece of cardboard.
then fill it up with paintballs while pushing the plunger down. Close the lid of your pod when done filling. (Note. Put your pod in the mouth of your hopper when you open it. They come flying out very fast. Depending on your spring size they should anyway.
Step 1: Create
All you do is hot glue the spring to the bottom of you pod and hot glue the cardboard onto the spring. Let dry.