find the thiniest mouse pads you can and cut them to fit the back wall and use a good two sided table to pad the back goal...use one in each of the goals (no need to cut or table here) and you will remove 85% of the sound and you can go to a ball that fits the table
probably the Warrior ball would be a good compromise...it is softer and so more quiet than most but it is a good size, keeps its shape, always rolls true, and will provide a good level of control for beginner players