We said that it’s simple, and it is: it’s a brick wrapped in a piece of paper.
If you don’t have a couple of bricks (and we didn’t), they turn out to be cheap at hardware and home improvement stores. These are “cement bricks” — red dyed cement– and cost about $0.25 each. Wrapping them up keeps them from scratching up your shelves and books, but also from depositing cement debris everywhere.
The geek part is on the outside.