What locality was meant by Trbontle, Yugoslavia? I have the notes taken on behalf of an illiterate coal miner immigrant for his affidavit for naturalization in 1936. In there it is written:  The issue is that no one from that area knows of any place named Trbontle. Closest guesses so far are Trbovlje or Trebnje. Trbovlje is not so far north from Veliki Cirnik, there is also a mine there. Trebnje is a bit south west from Veliki Cirnik and does not appear to have mining. The transcriber was probably not a native, but an American, so would Trbontle phonetically sound like either of those two cities as pronounced by a native of one of them? What cities could he have meant so I can try and find church records for their marriage?

I used the GOV (which now include placenames from most of Europe) and searched for Trb*. 3 possible matches to Trbontle were among the results:

Trbovlje, Slovenia (one of your finds, and probably best match),
Trbonje, near Dravograd in Slovenia, and
(less likely) Trbounje, in Croatia

Trbonje has its own church, according to Wikipedia, but the FamilySearch catalog does not include it.

You might want to search the GOV yourself, using other wildcard placements. Results are shown on a map as well as in a list, making it easier to see relative positions.

There may be other on-line gazetters which include Slovenia, but a cursory google search did not find any similar to GOV. There were some gazetters for download or purchase: you'll have to evaluate whether these resources fit your needs.