It should be noted that placing a portal "backwards" can put you inside a large boulder, too... and you can see out of it (on any faces that are uncovered on the outside) and shoot through it, too... although apparently unlike mobs, exiting is as simple as walking out of it.

Those big rocks in the plains are more useful than you may have thought.

https://youtu.be/ky9xo_UwQAA?t=426
(The plains boulder how-to starts at ~ 7:06)