Using game elements and mechanics in sports training holds great potential for increasing player enjoyment, but also introduces a risk of reducing training relevance. This paper describes a novel training installation for individual handball training, called "The Bouncer", and the design process behind three training games. In order to investigate how game elements can affect the training experience, we conducted a study with 10 experienced amateur handball players, eliciting responses regarding the training relevance of the games. Based on the study and our design insights, we propose three challenges that designers of interactive sports-training games need to consider: 1) Maintaining relevance when translating physical elements into digital representations. 2) Choosing an appropriate level of sensing as game input. 3) Introducing points in training exercises without reducing sport relevance. For the three challenges, we propose strategies to help future designers of training games.