Written in OpenSCAD and parameterized so you can adjust the size and exact mixture of tetrominoes that make up the book ends. The default is to render a single example bookend, but adjust to your hearts desire.

Samples printed in a mixture of PLA (Prusament Recycled) and PETG. I've printed with very low infill to allow space for filling with sand. The shapes can be glued together to add rigidity if required. It can be helpful to arrange them to have some squares sticking out away from the books to provide more purchase.

Warning: avoid arranging the tetrominoes such that the bottom row is full: there's a risk it will disappear.