You can just buy counterfeit anti-tamper stickers but if there is a switch inside the unit that flips a bit in some sort of write-once memory, then that would require removal of an entire chip and replacing it with another that may not be 100% the same. You can have a chain of trust in the system where chips will only talk to each other if they all spit out the right hash. Bury the SPI/I2C lines you use for this trust check within the PCB so you can't access it without drilling the card and add a layer of anti-tamper traces that trigger another tamper event if disturbed. Now what was just a quick install has turned into a whole PCB rework job where you are having to swap all the chips with a virgin set, assuming you can't get your hardware in there prior to final assembly.
Use an AES256 key from the factory to hash the chip's burned-in serial number and the time from the RTC. Lock out JTAG interfaces so once the chips are burned, they are inside their own fortress. There are a ton of ways to really lock down the hardware other than a shiny sticker and weird screws. Those keep people from breaking their own hardware, anti-tamper tech in chips keep out bad guys.
Use an AES256 key from the factory to hash the chip's burned-in serial number and the time from the RTC. Lock out JTAG interfaces so once the chips are burned, they are inside their own fortress. There are a ton of ways to really lock down the hardware other than a shiny sticker and weird screws. Those keep people from breaking their own hardware, anti-tamper tech in chips keep out bad guys.