I tried to use our Brother label maker, but the adhesive just couldn't stick to the Systainers.
When Peter Halle did the Sys labels for a group of "guinea piggies" I got some. And that's when I was shown the awesomeness of weatherproof polyester labels (the ones that Peter procurred and used).
But, I wanted bigger labels, to fill the entire "label area", with a bigger tool picture and (minor theft deterrence?). Also, I got a few more tools over the months and wanted to relabel as needed (ie. when the stock ones came loose). Two tools and an accessory kit I got "bare", adding the Systainer and the labels from scratch.
I use Avery's polyester, weatherproof shipping labels. I use the 5.5" x 8.5" size, and trim them to size. I now have a simple template in Adobe Photoshop I use to create consistent sized labels. When possible I put the stock label on the top lid, so I can tell what they are from above. Over time I'll probably change those, too (those stock labels just don't hold on so much...) These weatherproof ones... not a single corner has loosened over the last 4-5 months. I had to change one, and it took a lot of effort to get it off, leaving a great deal of sticky residue on the Systainer. Whatever glue they're using loves ABS.
Here's a sampling:
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