Oh. The fact that I don't know what backer rod is, but I saw what
looked like that at the store makes me think I don't know what the heck
I'm doing.

Backer rod..think POOL NOODLE material but 1/4", 1/2", 1" etc in diameter
(with no hole in the center) sold in rolls.

Roll out what you need and stuff it into the crack with the end of a ruler
or such.