Made a few modifications to the standard U3S case.
I haven't had DB9 cables around for years, so I nibbled the opening to fit a RJ45 jack. The connector is a panel mountable pigtail with one end cut off. For power I drilled the hole out to 22mm and added a 3D-printed Anderson Powerpole holder.
For the GPS, I used the case from a roadkill TV box. It already had a RJ45 jack, but sadly it was corroded, so I superglued in a RJ45 breakout. Now I can use a standard shielded Cat7 cable between the two. To prevent future-me from doing something stupid like plugging in a live Ethernet cable, I used the 4 wires not normally used in Ethernet.
Going through initial setup & calibration tomorrow.