In the traditional paper scrapbooking world, elements would be stuck together with two-sided tape. However, other types of fasteners could be used, like stitching, or paperclips.
In the digital world, no fastener is essential because everything will “stick” to your project, no matter what. So all the fasteners you can imagine can be created digitally, and used for decorative purposes.
PLAIN OR DECORATED
Most fasteners will tend to be fairly plain as they are quite small. You can use a color from your palette, a neutral color, or a texture that would simulare wood, metal, etc. You can choose something that is generic, or something that particularly suit your theme. For example, if you are using a camping theme, you might want a wooden button. If you are working on a baby theme, you can use a safety pin.
Since fasteners are not typically a focal point of a project, they just need to be a “normal” size. For example, a button is often half an inch, which would mean about 150 pixels (for a resolution of 300 dpi). You can make them larger and offer the user to size them down. That could be a welcome option. Just make sure you don’t make them too small since sizing up gives more issues.
Although many types of fasteners have a typical shape, you can change that a bit. A paperclip might be round, square, or in the shape of letters. You can certainly be creative and use a shape that is related to your theme. For example, buttons can be in the shape of a heart for a Valentine’s kit, or in the shape of an animal for a farm theme.
And remember that they will all be decorative, in digital projects so have fun.