The above oilcloth can be found at ReproDepot.com
Our daughter just started transitioning to solid food, and, while rice cereal eaten happily by a baby thrilled to be served 'big person food', but not yet wise to her ability to 'help' is pretty innocuous, I know that the days of sweet potato puree flinging are just right around the corner.
In an effort to be proactive, I have been checking out various websites for the best oilcloth with which to make a drop cloth. I can put it under her highchair at home, a restaurant, or at Grandma's, wipe it clean (even hose it off in the sink at home), and fold it neatly into the diaper bag.
I haven't started making it yet, so I have no 'how to' for you in the classical sense, but making a drop cloth is as easy or difficult as you want to make it. You could, for instance, simply order a yard and a half of a fabric you love (and it's hard to choose!), which will set you back around ten bucks. Oilcloth is pretty heavy, and doesn't necessarily need to be hemmed.
If you want to get fancy (and I know I do), you can order contrasting prints (remember: One Big Print + One Small Print = Happy Design World), and sew them together, making a reversible cloth with contrasting edges on one side. Easy-peasy!
Check out these lovely patterns from Mendels.com!
Some examples of fancy drop cloths found on OilCloth.com, a wholesale company.