iMovie lengthen background video ONLY in layered green screen project

OK, I feel like a real dufus, but sometimes I find iMovie incredibly un-ergonomic.:-)

I dragged both a file of me speaking against a green screen and a 10-second motion background video, into a New Project.

I correctly layered the two clips, with the desired effect of me now against the 'live' background.

However, the green screen file of me speaking is 10 minutes long; the background is 10 seconds.

So the background ends 10 seconds into my speaking video.

What is the "correct way" to set ONLY the background clip on infinite loop, so it continues always through the 10-minute speaking video? (Obviously selecting View>Loop Playback only loops the whole project.)

So far my only 'work around' has been to copy/paste the background loop 60 times till the background clip is as long as the speaking clip. But that's insane, there's got to be a simpler way to make one event clip loop while the other doesn't??? What am I missing?