I measured these to fit the typical debit-card style gift card.

I made several with different themes like Graduation, Golfing, Shopping, Autumn, etc. but these two Christmas ones turned out the best.

You could use snaps, velcro, buttons or ribbon for closure, all of them work, although I prefer a well-sewed on snap (then just cover up your sewing job with a decoration like the star on the tree or the snowman's hat blowing away)