I had this same problem about 2 years ago. I went to the dealership and they replaced the thermostat housing and o-ring. It was about $380. Then a year later it leaked again. I went there and they replaced it for free since it was the same problem within the same year. Plus the guy that did it before got fired because he did bad work on cars. But yeah, pretty much replace the o-ring and your set. But I went all out.
I posted about this before and alot of people had this problem.
Very easy fix and somewhat common problem , Big thing is DO NOT use sealer and make sure the head is clean (lightly sand with fine sand paper) and it should hold with no isuues , they have upgraded the o-ring that goes there
Had the same problem a few weeks ago with my 04' SVTF. Saw the leak from the o-ring at bottom of housing as your describing. Changed the thermostat housing, o-ring, and new thermostat, since I was already in there and it still leaked. Found the leak was coming from the sender at the top of the housing. Had to buy the whole sender assembly from Ford because they don't sell just the internal rubber o-ring that seals it. No more leaking...
Info came just a bit too late WD40. I bought a new housing and that didnt' fix it. I got it all back together and it still was leaking. I confirmed this time it was the silver plug. So Ford said I could not just buy the O rings. So I got a new plug and a new sensor. I'll get it back together tomorrow. Also gonna order a pack of those O rings.