It's a planetary sensor dome. All the 23rd century ships had those. It was't until the E-d that this changed.
Also, FYI there _are_ two models of the ambassador studio model, regardless of what you might see on some sites incorrectly attributing there being one model to a quote by Doug Drexler (including memory-alpha). So keep that in mind. The E-c model was only used in yesterday's enterprise, the remaining Ambassador model was used for the Yamaguchi, Zhukov and Excalibur. So keep that in mind when filtering out reference material for your work
Thankfully the movie has enough contrast (TSFS) that you can make out the depth of the greebling, but it does seem that under-saucer segment is pretty flat bar the twin tubes for the engines and the two recessed sections.
Great model, I don't like how the nacelle pylons of the belknap are mounted to the hull, but that doesn't detract from the quality of the work. Nicely done.