Hi Auster,
If you are using the recommended FTDI USB to RS232 cable, you will find that if you plug it into Port 2, the plastic ‘strain relief’ on the USB plug will slot nicely onto the lower case, effectively holding the plug into the port, though you might like to gently open up the semi-circular recess slightly to ensure a good fit. If you want to put the ‘upper case cover’ back, you need to open the ‘upper part of the hole’ on the cover large enough to let the cover drop over the strain relief and slide back into place without snagging.
Regards
Peter