How to design a narrow band pass filter without DSP toolbox
The steps that I would follow to handcraft a bandpass filter are these:


Figure out how wide (in Hz) you want the filter to be.
Create a low-pass filter with half the one-sided bandwidth as your desired filter bandwidth.  You can calculate the sinc function with basic functions, since $sinc(x) = \frac{sin(\pi x)}{\pi x}$.  You can adjust the bandwidth by adjusting the time scale in your sinc function.  If n is the set of all integers between -N and N, inclusive, and you make your filter taps equal to $sinc(bn)$, then the filter's normalized bandwidth will be equal to $b$.
Modulate the lowpass filter to the center frequency you want for your bandpass filter.  You do that by multiplying it, element by element, with a sine wave of that frequency.