How can I get more Fourier coefficients than the cfit tool?

Hi, I'm new to the community, any help would be greatly appreciated!

I'm working on a project which calculates the Supersonic wave drag of an aircraft from its cross-sectional area distribution. I have discrete data for cross-sectional areas at different points along the aircrafts length. This line gets differentiated, and the fourier coefficients of the first derivative are used to calculate the drag force.

The problem is the curve fitting tool only allows up to 8 fourier coefficients, which gives a poor fit (see below)

As a result, the overly smooth line is leading to an underestimation of the drag force.

Is there a way to get more fourier coefficients for discrete data please? I've seen the fourier_coeff function, but this requires an expression for the function of line which I do not have given that it is discrete data. It also can't compute from a cfit object.

Thanks in advance for any help.

Ed