Hacker News Reader

With the default ext4 filesystem, mounting 'ro' doesn't actually prevent writes to the disk. For that you need 'noload' too, to prevent journal replay. This can result in a broken filesystem if it wasn't unmounted cleanly, although assuming it's 100% read-only it shouldn't do any permanent damage (perhaps enforce read-only access at the block device level to be sure).

You might like: https://www.kanguru.com/products/kanguru-ss3

Using an SD card (or micro SD in an adapter) connected to a USB reader might meet your needs. You can then use the SD write protect switch.

I thing I learned only recently is that the write protect switch on the SD card is not an electrical switch connected to anything in the SD card itself: it just hits a lever in the SD socket that opens a contact closure and it's up to the system (hardware and software both) to bother to look at it. So on many systems the write protect switch doesn't even work.