When I was collecting sports cards, little did I know that a lot of the packs I opened contained something I could have used now. The thicker packs of some brands , especially Donruss, in the early 2000's most of the time contained a thick dummy card, a nice pure white piece of cardboard (pic no. 1), which was used to make the purchaser think that the pack possibly contained a memorabilia card,(one that had a swatch of jersey or slice of bat, possibly with an autograph). I had thrown away hundreds of these "useless" pieces of cardboard but thought that I might be able to use them someday, so I managed to hang onto a few dozen of them.

     When I took up oil painting I noticed the small size of some of the artwork for sale on different websites. Some were called "art cards" which were the size of a baseball card. That turned on the "idea switch". My project began.
      The first problem that I ran into was, "How could I keep these little canvasses from moving around while having them usable with my easel ?". I found a scrap of thicker pressed board type of paneling which was the size of a larger canvas and could be held still by my easel. On the paneling piece I made a bracket around 3 sides of a blank card by using a household staple gun. I used a few blank cards on top of each other to gauge the depth of the staple penetration. I wanted the staples to be used to hold the card still while painting on the card, and to be not a problem when removing a dry and completed piece of artwork.(see pic no.2)

    Making multiple brackets enables you to work on many of these small paintings at one time. When making the brackets put your paneling piece on a surface that is scrap. You wouldn't want to mark up a nice table by having a staple go through while you're making your brackets.
    Make a layout of a few landscape format and a few portrait format brackets for working on , or just make a couple of panels, one for landscape and one for portrait formats. Since you don't have to touch your wet artwork,you can change your bracket panels to work on another set of miniatures while your other set dries.
    Before I started oil painting on the cards I covered them on the painting side with an acrylic paint to give a surface that had a little tooth along with a layer that would be between the oil paint and cardboard. After the acrylic paint dried I proceeded with oils.
    Since these miniature paintings are the size of a standard baseball card you can use the card sleeves and thick card toploaders made for sportscards to hold your artwork after it has completely dried. These can be found at larger department stores or sportscard stores. (see pic no.3).
    For a more permanent setup use a piece of plywood or chipboard and plastic, nail in cable clips instead of paneling and staples.

extra notes: (added after first publication) I first found the blanks in 2001 Playoff Absolute and Donruss Studio brands of baseball cards. The Donruss, Playoff, and Leaf brands all contained these blanks through 2005, (all from the same main manufacturer). Through a search on Ebay, I found that these brands are again found in baseball cards, though I don't know if they contain the white thick blank cards. I also found on Ebay a product called "illustration art boards" in the ACEO size. They also have this size in masonite, 140lb watercolor paper, canvas, and colored matting. Also, sports card thickness is measured in points, and plastic toploaders are described as being able to hold a certain pt.card, with a 55pt. size being a typical thick card toploader size.