Just saw this clever doohickey. It's a plastic board that slips over your bass's fingerboard and nut to turn your fretted bass into a fretless.   Almost instant - you have to take the strings off or at least detune them til they're floppy. Could be done between sets if not between songs.   I wonder what it does to the intonation though - might be a problem, especially on a fretless where you are using the frets as markers.