Based on what you've given us to go on, the instrument with the resonant frequency of this cave you visit would be an identical cave. :-)
Before you start thinking about instruments, materials, or tuning, you need some ballpark as to what this resonant frequency actually is.
The most reliable way to determine this would be to bring a tone generator (like a laptop) and amplifier into the cave, and sweep through a broad range of frequencies until you find the one that hits. But you don't have to go to those lengths if you have some information to start with:
- Can you hear a resonant frequency when you are in the cave, perhaps from the sound of wind? If so, then the frequency is within the range of human hearing, i.e. 20Hz-20kHz.
- Can you match the resonant frequency using your voice? If so, then the frequency is within the range of the human voice, approx. 80Hz-1,100Hz
Once you have your target frequency, then you can go about finding/tuning an instrument. Honestly, in the percussion world of found instruments, this is mostly just trial and error. If you are making your own instrument, there are simple formulas for the resonant frequencies of a cylindrical tube, which may be useful.