Antennas interact with their surroundings a lot. If you intend to integrate the antenna into the plastic case, I may not be able to suggest something practical without studying the available volume first hand. This is pretty hard to do via a forum like this. My suggestion of an ideal monopole requires that you place it on top of the tennis ball machine. This is because the interaction that I spoke of is most powerful to the sides of the vertical element. This means that if you place a vertical monopole alongside anything and especially metal, its performance will be degraded. However, if you place a monopole such as this on the very top, in the clear of any metal, the "ground plane" at its base isolates it from metal underneath, so no degradation occurs. The suggestion of a monopole, therefore, requires that you put it up on top with the vertical wire poking out from the top of the machine.
If that is acceptable, then I'll describe it in a bit more detail. Now, I don't know if your receiver uses a coaxial cable to attach its antenna to the circuit board. Possibly not and in this case we have to do things slightly differently. If there is no coax joining antenna wire to receiver board, then we have to place the receiver board up at the top of the machine and extend the antenna wire vertically, straight up, a distance of about 138 mm from the surface of the receiver board. The "ground plane" will be made of four wires, called ground radials, all 138mm long that are soldered to the ground on your receiver pcb (close to the bottom of the vertical wire). The four wires are dressed outwards horizontally, or better, bending downwards about 30 deg or so, and in four different directions like the points of the compass. The length of these wires are to be trimmed so that the distance from the bottom end of the vertical radiating wire to the tip of each horizontal radial wire is about 138mm. That's about all there is to it.
If there is coax, post again and I'll suggest how to handle that.
I'm guessing that you may not be happy with having an antenna wire stick up above the machine. There are many other ways to do an antenna, but that is the simplest and most reliable, electrically speaking.
this link shows a simple quarter wave vertical antenna. Scroll down to the second image, which shows a version fed by coaxial cable.
**broken link removed**