The design of traditional musical instruments is a process of incremental refinement over many centuries of innovation. Conversely, digital musical instruments (DMIs), being unconstrained by requirements of efficient acoustic sound production and ergonomics, can take on forms which are more abstract in their relation to the mechanism of control and sound production. In this paper we consider the case of designing DMIs for use in existing musical cultures, and pose questions around the social and technical acceptability of certain design choices relating to global physical form and input modality (sensing strategy and the input gestures that it affords). We designed four guitar-derivative DMIs designed to be suitable to perform a strummed harmonic accompaniment to a folk tune. Each instrument possessed varying degrees of `guitar-likeness', based either on the form and aesthetics of the guitar or the specific mode of interaction. We conducted a study where both non-musicians and guitarists played two versions of the instruments and completed musical tasks with each instrument. The results of this study highlight the complex interaction between global form and input modality when designing for existing musical cultures.