Add viewports in Layout (paper space)

I'm trying to set up my drawing sheets on a layout tab but I can't find any way to insert a viewport once I'm in paper space. That's how I'd normally frame the model geometry onto a sheet and lock it to a plot scale, so at the moment I end up finishing the sheet composition in another program before I can export anything. Being able to drop a rectangular viewport, set and lock its scale, and pan the model around inside it would make this genuinely usable for real drawing sets — it's honestly the main thing stopping me from doing the whole thing here.