Hi Will,
I don't think we can force the browser to stay in a portrait layout - that's handled at the app level. Since the SmartBuilder lesson lives inside the browser (Chrome, Safari, etc), we're subject to its behavior.
However, if you don't want to create multiple layouts, we can detect when we're in a different layout, and inform our learners to rotate their phone (or device) back to a portrait orientation. The topic is
discussed in more depth here, but here's a video that walks through how to set this up with zero actions:
https://share.smartbuilder.com/public/support/please-rotate-your-phone.mp4
And here's the sample lesson you can play with on your phone -
https://lrs.smartbuilder.com/l/u633lw856eb9
Note that with this method, you're pretty much locking out desktop users. Another possible solution is to just create the one phone portrait layout, and if people want to rotate their device and see it super zoomed in and scroll up and down, that's their choice!