Schematic For Marine Battery Charger Where Can I Find A Circuit Schematic For A Two To Four Pole Bessel Filter?

Where can I find a circuit schematic for a two to four pole bessel filter? - schematic for marine battery charger

I have two four-pole Bessel filter with a frequency of 5 Note to 10 kHz. I need a plan that can start the work, and maybe even a formula for the cutoff frequency.