פירוש על עבודה זרה 38:11

The mishnah prohibits participating in building the cupola because that is where the idol is placed. This is too close to abetting idol worship.
If a Jew transgressed this law and did help build the cupola, he may make use of the pay he received.

The Talmud argues the R. Yohanan’s ruling is overly obvious. R. Ishmael and R. Akiva will argue later in the tractate over idols themselves—are they prohibited before they are actually worshipped. But they would both agree that if the item is not itself an idol, it is not prohibited until it is actually worshipped.