UPDATE: Here is a source for the tin used below.
I spent some quality time yesterday with a soldering iron, ohmmeter and electric drill and ended up (finally) with the GPS module of my Ultimate 3S now enclosed and remoted to the window sill of the shack.
Previously, the GPS was wired directly to the U3S and sat beside it on the shack desk. This necessitated leaving the U3S housing opened which in turn contributed to a bit of drift on received WSPR reports.
I tried to find an all-plastic housing (since metal would attenuate the GPS signals received) of the appropriate size - and then I remembered a small box I bought from the Four States QRP Club almost 10 years ago with the idea of doing something with it some day.
"Some day" took a long time getting here but the case is perfect - a metal bottom and sides (for drilling mounting holes and serial cable access) with a clear plastic top enabling GPS reception and the ability to see the three status LED's.
I also added a cheap 12V to 5V converter internally to the U3S allowing it to be fed from the shack 12V bus rather than the external 5V supply I had been using.
On the way to me is Hans' 5-watt RF amp for this with my ultimate goal being to use the U3S as a 6-meter WSPR beacon.