There has been a lot of work on these currents over many years, most of it well above the molecular level. It would be very surprising if there were not already at least one paper that presents a model for the cAMP dependence of h currents. Indeed, a preliminary search of the Internet for
hcn camp dependence model
quickly turns up this paper from Siegelbaum's lab
Activity-dependent regulation of HCN pacemaker channels by cyclic AMP: signaling through dynamic allosteric coupling.
Wang J, Chen S, Nolan MF, Siegelbaum SA.
Neuron. 2002 Oct 24;36(3):451-61.
which presents such a model. The vintage of this publication suggests that there are many more yet to be found; it would probably be more fruitful to do such searches with PubMed.
A search of ModelDB https://senselab.med.yale.edu/ModelDB
for entries that involve h currents finds 219 models, each of which includes its source code. Indeed, one of those entries (19176) is a NEURON implementation of the model described by Wang et al.. You might want to examine the others to see if any explicitly represents cAMP effects.