This 6-PAGE PLASTIC BIFOLD WALLET INSERT is serviceable, although I would have preferred an insert with slightly larger windows made of heavier plastic.  The windows hold cards measuring a maximum 2-1/8" x 3-3/8".  Standard plastic credit cards fit, but some paper cards (e.g., Medicare cards) need a bit of trimming. Each window easily holds two or more cards.

This product listing is for an insert with SIDE LOAD windows.  This means the cards go into the windows with the narrower (2-1/8") end leading.  The insert fits a man's standard bifold wallet--the type of fold-in-half wallet that men carry in the inside pocket of a jacket or the back pocket of a pair of pants.

Because credit card window inserts are so hard to find, I was glad to find these inserts for sale on Amazon.  At $4 for two inserts, it doesn't really matter that the inserts are aren't absolutely ideal.