What cardboard could I use?

[MrsFlannel]

To make a frame for a small mirror? I don't want to get into wood or hardboard...but want a thick durable cardboard which I could cut with a craft knife? Is there such a thing please?

What about a thick cardboard packing crate, the sort that's used for moving house? You can buy them used or new from removals companies, and if it works out well you'll have loads left to make more frames.

Would the edges be corrugated in appearance though? I need the edges to be smooth...and paintable.

Would something like Bristol art board work for you? Perhaps take a trip to an art and craft shop?