Positioning of a tiny voice

[Noeck]

Hi,

I would like to add small notes into an already populated staff.
It is the right hand of a piano staff and I don’t want real voices with
their shifts. The tiny font size distinguishes the "upper" voice
already. It should have the least impact possible on the "music" voice.

Here is one example measure:


\version "2.19.84"

upper = \relative {
  \tiny
  \stemUp
  s4 b''
}

music = \relative {
  \key e \minor
  \time 2/4
  b'8 <b, e g>16 c' <b, e g b>8 <b e g>16 b'
}

\new Staff << \upper \music >>


How to move the b'' exactly on top of the chord below?
What to do about this warning?

warning: this Voice needs a \voiceXx or \shiftXx setting
  b'8 <b, e g>16 c' <
                     b, e g b>8 <b e g>16 b'


Cheers,
Joram