A simple categorical model of predicative polymorphism

I'm looking for a *simple* categorical model of a predicatively polymorphic
lambda calculus, that is, a model which avoids the complications needed for
I just need a model of an explicitly typed version of Core-ML (including

I found the following, promising reference:

W. Phoa: A simple categorical model of first-order polymorphism (submitted)

Unfortunately it seems that this paper has never been published and the author
left academica.

Furthermore, it seems to be folk-lore that polymorphic functions are natural
transformations. It would be nice, if the model expressed this formally.

Any pointers to literature are highly welcome.

Thank you,

OLAF CHITIL, Lehrstuhl fuer Informatik II, RWTH Aachen, 52056 Aachen, Germany
             Tel: (+49/0)241/80-21212; Fax: (+49/0)241/8888-217
             URL: http://www-i2.informatik.rwth-aachen.de/~chitil/