Obviously we need open source designs.
Perhaps the easiest way would be a Raspberry pi set up with an opto isolated CGA/EGA/VGA/SVGA capture that could be viewed via the internet? (I mean, we're probably talking systems still running MS-DOS or Windows 98 running these systems)
I'm told a box with two optical NICs in it with only one fibre in it, TX->RX, works quite well, plus a bit of software to wrap around it.
But these sorts of organisations love a bit of bureaucratic regulation, and "certified" devices and that sort of thing, so there's probably not much of a market for a data diode without the paperwork.
On the opto isolated idea - I think there was a QR-code and camera based data transfer tool posted on HN a few days ago!
To DIY it you need three NICs (or an optical isolator, which is honestly more work than it’s worth) because the transmitting NIC needs to see a link up before it transmits anything. That link is most easily provided by the second port on the transmitting side.
Developing software to reliably move data across the link, and ensure that it’s properly formatted etc will still be your problem.