The steam deck already does limit charging to 80% after being plugged in for an extended period of time so the battery cells will still have a charge but no be in the harmful range.

Saw this interesting 3d print design that might inspire you

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I read fallout 2 in your comment and had to reread again lol

I read Fallout and was very confused.

I tried 3 because of the good memories. Made it as far as toy story and lost interest.