I use a Boss OC-3. I originally bought it to cover B string parts when I decided to switch back to a 4 string. And for that it does the trick, since I had always used those notes very situationally even when I was maining a 5 string. However, I'm quickly finding all sorts of fun uses that I never thought of before buying it. Guitar solos are a great spot to use it for example. We cover ska songs sometimes, and the walking bass parts can sound annoyingly normal, so I play them up an octave and turn on the OC-3. Instant video game boss fight style cover!