Seven term Blackman Harris window
In this document you can find the coefficients of a seven-term Blackman Harris window. Ignoring the bizarre notation, it seems like the window is defined by

$$w[n]=\sum_{k=0}^6a_k\cos\left(\frac{2\pi kn}{N}\right),\quad n=0,1,\ldots,N-1\tag{1}$$

where $N$ is the window length and the coefficients $a_k$ are given by

$$\begin{align}
a_0&amp;=0.27105140069342\\
a_1&amp;= - 0.43329793923448\\a_2&amp;=0.21812299954311\\a_3&amp;= - 0.06592544638803\\a_4&amp;=  0.01081174209837\\a_5&amp;= - 0.00077658482522\\a_6&amp;=  0.00001388721735\end{align}
$$

Unfortunately, they do not mention where those coefficients come from. Somebody must have run an optimization routine to minimize the sidelobe level for a given mainlobe width.

EDIT:

I think I found the relevant paper here. This is a corresponding Matlab function.