I usually only make things like a 20th fret, extra strings or weird scale lengths if I am copying an existing instrument but here is a just-finished 20 fret on a standard concert guitar. I tried to make the fretboard extension as aesthetically pleasing as possible, have a look. I used circles of a specific radius to trace the contours and I think it shows. You can compare it to the solutions proposed by others.

M. Kohno 1999

M. Aracama 2011

F. Santiago 2011

R. Moreno 2003

F. Sheppard 2009

Arias 1900

T. Humphrey 1990