Now that AutoCAD LT has the ability to create non-rectangular viewports, I thought I'd share a way to create viewports with "holes", or empty spaces.
Let's say you want a triangular viewport with a circular hole in the middle.
The first step is to turn the objects into regions with the Region command, available on the Draw panel of the Home tab. Just start the command, and select the two objects. When you're finished, they don't look any different, but when you select them you'll see that the grips have changed, and if you try to grip-edit the shapes they'll move instead of stretch.
To get the hole in the middle, you'll use the Subtract command, to subtract the circular region from the triangular. This command is a little sneaky, since it's only available at the command line. (Of course, you could always put it on a button if you want.) You'll first pick the object to subtract from (the triangle), and then the object to be subtracted (the circle). Again, at first glance, nothing seems to have changed, but when you hover over the region boundary you'll see that it's now one object instead of two.
Now you're ready to turn the region into a viewport. To do this you'll use the "Create from Object" function from the Viewports panel of the View tab. This enables you to convert an existing object -- a closed polyline, an ellipse, a spline, a region (hint, hint), or a circle -- into a viewport. Just start the command, select the region, and there it is.
The region has become a viewport. You can pan, zoom, and set the scale just like you would with any other viewport. The only catch is that you can't edit the boundaries like you would with a polygonal viewport. To change it, you pretty much have to erase it and start over. (Sorry.) It can still be pretty handy, though!