When using multiLoader to load structures, it doesn't seem to work if the id of my div is not "viewport", even if I pass a different id to multiLoader. It seems that if a stage with custom id does not exist, it is not created either.

In NGL.specialOps.load, below the comment "- check if there is a stage", changing
NGL.stageIds[myData.id] = new NGL.Stage('viewport', {backgroundColor: myData.backgroundColor}); to NGL.stageIds[myData.id] = new NGL.Stage(myData.id, {backgroundColor: myData.backgroundColor}); seems to solve this issue.