If you made wall of bricks it can be said "you сложил wall", because you "added brick to brick" in process.
The idea of "сложившееся мнение" is the same: some monolithic opinion was made from different facts and thoughts. Facts and thoughts was "added" one to another to form this opinion.
So, it is just some way to say "opinion that was created (by joining some things in whole picture) before".
Also valid: "сложившаяся практика", "сложившийся порядок вещей" and so on.
The only thing that this adjective impiles: process of creation was complex, involving several steps or parts.
So, many ways of translation are correct, because it doesn't matter in this case.
P.S.
Yep, as was said above, russian "to add" (сложить) is derived from "to put together" (сложить).