This is a really quick idea.
I cut a piece of card that was 10cm or 4" square.
On 3 sides you mark the 5cm or 2" Mark.
Score it from the centre to both edges.
This gives you the triangles on each side.
fold these down to the back.
Cut a piece of card that is 7.25cm or 2 7/8"
Cut it in half from corner to corner, to make two triangles. You only need one.
This is then glued to the back of the bookmark to cover the two folded over corners.
You then just decorate it.
I was lucky as my A mark a my envelope maker is great for the scoring of the corners.