I make my own game and KFTM cards with 3mil
matte pouches (glossy pouches are useless for this purpose). I binder clip multiple cards together for multi-game days and adequate stiffness, so 5mil is probably better if you only have use for a single card. While it isn't a pen, I believe a pegcil is the best bet regardless of conditions. Clips right on the card, it will always write, and has no chance of bleeding from inclement weather or sweat. When cutting/designing, most of the popular manufacturers leave a small margin of laminate outside the paper which helps avoid deterioration of the paper, or bleeding of the printed ink inside. I learned this from experience, as the cards pictured which were my first iteration now have ink bleeding around the edges.
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