Rotosound TruBass will give a more upright sound as will La Bella black nylon tapewounds, but the latter are very hard to get in the UK right now. TruBass are unusual to the uninitiated as they are very thick (65-115), plasticky when unamplified, and probably need higher action than many would prefer. They won't sound exactly like an upright but they will sound more like an upright than normal flats or half-rounds (esp when plucked around the end of the fingerboard) and will certainly sound enough like one to fool some people.