How can I actively provoke data rot on a storage unit?
If you're happy to use a data storage unit this small, then a UV-EPROM would be very easy to rot. They are still available, online auction sites have them, for not too much money. Even a 27C400, 4 Mbit, is only a few pounds.
Of course they don't store 'files', just raw data, so you'd need some form of external memory controller. Perhaps use an Arduino to address it, and represent its data as a file. FWIW, some CircuitPython modules can be read as USB memory, so could read the data from the EPROM and write a file to their local storage area that represents it.
When I first came across UV-EPROMS early in my career, more decades ago than I care to remember, I was concerned about the recommended write and erase times, so programmed one, then verified it repeatedly, with one minute in the eraser between verifies. It took a few minutes for the first bit to drop out, then most went between 5 and 10 minutes, needing more than 15 minutes to erase the last bit. Quite a range.