Use mature compost near the top few inches of the beds. This will give your plants and soil microbes lots of nutrients that they will need throughout the season.

But mostly use a lot of filler material like cheap plain potting mix materials to cover and fill most of the raise beds.

Keep in mind that compost will shrink and disintegrate over the seasons. Most potting mix materials will last for decades!

Happy Gardening!

sure, but the volume will shrink over the season and will need to be topped off every time a plant is pulled. Other than that composted anything makes for a fine filler for a raised bed.

Don't forget to add dirt...plain old everyday dirt. Good source of minerals and other good stuff. And sand if you can get some. Even some clay for moisture retention. Last but not least some good old elbow grease. :)

Soil has two parts, the mineral component and the organic component and you need both for a good, healthy soil. Adding organic matter to the mineral portion of your soil should be all you need to do, unless your are raising the soil level more than several inches. Generally, buying some of your local "topsoil" is a waste of your money since "topsoil" is simply the top 4 inches of soil that is scraped off the land and most likely will just be what you already have.