Take a HDPE bottle (recycling #2) from your trash or dumpster dive at the recycling center. Â Cut the top off. Â Smooth the cut edges with fine sandpaper (or not). Â It will weigh 15 to 25 grams. Â Won’t burn your lips. Â Slightly less hot to hold than a metal cup.
I recommend HDPE because it stands up to dish washer use and after that hot water wash with a caustic detergent and the hot water rinse, there won’t be any shampoo, conditioner, mustard, whatever left inside. And it will be easy to clean up between trips.

Here’s a 16-ounce Hydrogen Peroxide bottle that sells for $1.09 (new, with H2O2 in it) at Walgreens. Â Use the H2O2 to clean some blood-soaked clothing or something and cut it down to a 14- or 15-ounce coffee mug.

The lip will be a little floppy on some bottles once cut, which is why I save the H2O2 bottles (after cleaning up the blood) because they are thicker than most consumer products bottles. Â Also, fuel additives, HEET, and such also come in somewhat thicker-walled bottles, but then you’ll have to believe the chemical engineer at just how good dishwasher detergent is at removing oils (you know, doing what it does for your dishes!). Â And those have the advantage that no one else will drink from your mug that once held toxic chemicals.
Advanced repurposing: you can cut a band from the container and jam it on as a second layer near the lip to stiffen up the opening. Â And/or cut some strips and fashion a handle if that’s how you prefer to drink your coffee.