A one-time pass is still uncrackable.

Take your secret. Write it on a piece of grid paper. Fill EVERY FIELD on another piece of grid paper with random data. Transform every character on the first piece of paper based on the corresponding characters on the second piece of paper and write the result on a third piece of grid paper. Then burn the first piece of paper.

It's not possible to read the 3rd piece of paper without the 2nd now. You also obscure the length by filling the second piece of paper with data.