HELP!!! TENNIS PRODUCT!!!?

i just bought a fill and drill ( A base with a long rubber band connected to a rubber band where you can hit it down and back to yourself with the rubber band bringing it back) When i bought this it didn't come with a ball that had the string in it….. So what is the best way to get a tennis ball on there where it will STAY ON and WILL NOT RUIN BALL!!!!
Please help

get like rubber ball