I'm currently using two piezos on the bottom edge of the soundboard, mounted on the inside, with the stereo jack being inside the handle hole in the back.
Filtering is a great option.
I'm not sure how many people actually use effect pedals and preamps, but there are many inexpensive EQ pedals, which allow you to dial down the frequencies which contribute to "knock" and hammer noise. Normally they don't use a large amount of power, so I use a battery-powered one. If that matters to you, then do a search to be sure the pedal can use a battery. The MXR 10-band one and the tiny Mooer EQs only take an adapter, but the simple Boss and Danelectro pedals can take both.
Getting even fancier, one can use a parametric EQ to zero in on the frequency range which is knocky, and notch that out. Some of the acoustic guitar preamps allow this as well. My Boss AD-5 has a sweepable anti-feedback notch filter, and I'm sure it's not the exception. The AD-5 also allows emulating the natural resonance of the instrument body, so it doesn't sound so one-dimensional.
Not everyone is looking for the piano/cimbalom bass range, so rolling the entire bottom end out might work for you. However, if you're looking for preserving the whole range of your instrument while just getting rid of a few frequencies, an EQ can be a good solution.
Whatever you choose, good luck!