Hi, I just discovered this forum, as I'm a guitar player who is interested in picking up a ukelele for a new challenge. But this particular post is right up my alley. Complex jazz chords, yay.
The maj11 chord is a weird bird. Going by the way other maj chords are built, it should be 1 maj3 5 maj7 9 11. In Bb, that would be Bb D F A C Eb. There are several problems with this chord though. You've got two pairs of notes that are semitone intervals (Bb,A and Eb,D) and you've got a tritone between the A and the Eb. This is why maj11 chords are almost always altered somehow. Sometimes the 11th will be sharpened, sometimes the third will be omitted - which really makes it a sus4 instead of a major chord.
You could try playing one of these:
Bbmaj7 - 3 2 1 0 (Bb D F A)
Bbmaj7sus4 - 3 3 1 0 (Bb Eb F A) (3rd omitted)
or less likely:
Bbmaj7#11 - 3 2 0 0 (Bb D E A) (5th omitted, 11th sharpened)