In December 2019 I heard from Jerzol from Poland who had added a SATA drive to his t620. You can read about that here.
In January 2021 I heard from Max from Italy who had added a VGA port to his t620 Plus. You can read about that here.
In February 2021 I heard from Pawel from Poland who had installed two 2.5" SATA drives into his t620. You can read about that here.
In May 2022 I heard from Evan from the US who had fitted a 2.5" SSD to his t620. You can read about that here.
In October 2023 I heard from Marco who had fitted a second ethernet interface to his t620. You can read about that here.
In October 2023 I heard from Marco who had fitted a second network interface to his t620 in place of the serial port.
He sourced his mPCIe card from eBay. The one he selected was based on the Realtek RTL8111, the same chip as used on the t620 motherboard. I found the same thing on Amazon.
What you get with these expansion cards is the mPCIe card, a length of ribbon cable and a RJ45 socket mounted on a back plate. I note that with these add-on interfaces quite a few have cables that plug in vertically. With the one Marco selected the cable runs horizontal which no doubt makes it easier to fit in the t620.
You will need to do a little bit of metal work to get the socket to fit - for example cutting the back-panel mount down to size. You can also see that Marco has removed the extra material from the mPCIe circuit board - cutting it along the line of pre-drilled holes.
Finally he was able to tuck the cable out of the way. I guess if you have a spare connector to hand you could always shorten the ribbon cable.
Going back the original photo Marco used super glue to keep the RJ45 panel in place and the ubiquitous Dremel tool to modify the plastic back panel cover. The screws he used for mounting the mPCIe card were M1.6x4mm.