My thumb "floats", which according to my definition, means that the RH thumb rests on the E or A strings, a pickup, fretboard, body of the bass, ... according to desired sound, shape of the line etc. I play a 4, and floating is great as long as I have a string to rest my thumb on. However, when it is time to play on the E string, my techique gets a bit less solid, as I can't count on there being anything to rest my thumb on. I'm considering having a thumbrest made and installed on my bass, and have a few nontraditional ideas about shape. When I play on the D string, I stick my thumb between A and E strings, with the weight of the hand on mostly on the A string. My thumb is ready to provide a classical guitar style thumb pluck on the A string, while the side of the thumb mutes the E string. The ultimate thumbrest for me would be positioned and feel like a B string on a 5, and would permit me to rest my thumb on E string / thumb rest just like described above when I am playing on the A string, and give me a consistent anchor point when I need to play on the E string. Anybody do anything like this? I would assume a good luthier could fabricate such a thumbrest. Or maybe I should try a 5 again Shapewise, I am imagining a hunk of plastic that in profile has a low edge and a high edge. The high edge would be sanded down to the approximate radius of a B string. The rest would be positioned so that the high edge is approximately where the B string would be, and the downward slope to the low edge would be such as to avoid string contact with the E string when digging in. The profile might actually be a right triangle.