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 over your paper and use it to “cheat” on your dimensions.
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.