I just go for the BIG BLACK X's in thick magic marker across the product, with the word gluten contaminated if there is enough room. I also have some of those washable window markers that work on glass storage (and maybe plastic, too). Thankfully, this doesn't happen to us a lot, except when we meet with groups for camping or at the family cabin, and some of our food might get inadvertantly contaminated. I travel with a portable refrigerator, too, to keep our food separate.
How about... sending out for some of those Mr. Yuck green stickers and plop those on his gluten food leftovers? I suppose he wouldn't find that any more humorous.
Anyone talented in computer graphics? Maybe we could have a contest for creating the best gluten contaminated sticker...
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Cara