I admit I blatantly copy ideas I find on the web. I wish I remember where I got this one. (edited to credit:http://sketchbook.cheapjoes.com/2013/10/don-getz-to-journal-final-words-from-the-u-s-odyssey-journaling-tour/) His is bigger, mine is cheaper.
This is a Dollar Tree picture frame for a dollar. It had a magnet on the back for fridge mounting, but it was easy to remove. It fits perfectly on my cheap 4×6 Watercolor pad that I carry around for practice.
I slide it off the pad and use a vis-a-vis marker to trace the object in the distance. Then you move the frame down and hover over your paper and use it to “cheat” on your dimensions by drawing behind it.
I may add magnets or velcro to attach a light palette. See this guy for the basic idea. http://freekhand.blogspot.com/2011/06/going-hi-tech.html That would make it a two purpose tool.