A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Noell Wilson
Date: 2017 Mar 17, 08:19 -0700
To answer your question: I don't think that you can get quick, easy, and cheap in one package.
To meet your stated goal of backup if the GPS fails, I'd recommend Geoffrey Kolbe's Long Term Almanac 2000-2050. It's available for about $30 and, with your Mk 3 sextant and a decent watch, will get you home. This meets the "cheap" and also a "small" criteria.
Easy? Not as much as some much larger sight reduction books and not near as easy as some self-contained electronic methods.
Quick? No. Geoffrey's book requires several extra steps to produce the same information that is readily available in the annual Nautical Almanac. But, if this is really a backup, you'll probably forget to buy a Nautical Almanac each year and you should have time at sea.
Geoffrey's Almanac also confines you to using just the sum and stars, but, it will get you home and many people with extensive equipment just use the sun.
Having said all of this, I'd recommend practicing a bit before shoving off.