This is somehow embarrassing, but I post it anyway, hoping that it may help others.
Although it looks easy, I had trouble assembling the mini pan-tilt kit from adafruit, specifically the bottom servo. I just couldn't figure out how to fix the servo with screws to the main arm, since there are no holes for screws (unless I drill them myself). Also, there is no guide anywhere to be found (uncharacteristic for adafruit), nor explicit photos of the assembled parts. I thought that there must be a trick, so I bought the assembled version as well. Guess what? The "trick" is obvious, and I missed it because I was focused on using the screws! The bottom servo must be inserted in the 2 tracks on each side of the two symmetrical parts that make the arm, no screws needed!
When assembling the kit, the easiest way is to start by attaching together the 2 symmetrical parts of the arm, with the servo between them. (In the photo below, i was too lazy to completely dis-assemble the 2 halves of the arm, I only pulled them apart enough to fit the servo in between, with servo's bracket in one of the tracks.) Notice the servo's bracket snug in the groove.
See? No screws on either side of the servo! Because they are not needed!