[XeTeX] Maths font for Adobe Garamond

[Thanos D. Papaïoannou]

Dear all,
I would like to have a maths font aesthetically compatible with Adobe
Garamond to use with XeTeX, a problem already solved for LaTeX:
http://www.lagom.nl/latex/agaramond.html using a certain "MathKit"
utility. Unfortunately, the files provided in that solution fail to
work on my system, as I can neither find the precise font used by the
author, nor do the variants of Garamond on my system come in the
pfb-afm format. Instead:

~> ls .fonts/AGaramond*
.fonts/AGaramondPro-BoldItalic.otf  .fonts/AGaramondPro-Italic.otf
.fonts/AGaramondPro-Bold.otf        .fonts/AGaramondPro-Regular.otf

Do you have any suggestions as to how one would proceed to generate a
maths font as in the solution above beginning from a batch of *otf
files?
For example, does anyone have experience with MathKit for OpenType fonts?

Thank you in advance,
Thanos