A letter that will fit.

1. Print this page at 100% scale. Do not use “fit to page” — it will shrink the sheet and the folds will land wrong.
2. Write on it. The ruled side faces in.
3. Cut along the outer border.
4. Fold along the vertical centre line, then along the horizontal one. It becomes 100 × 65 mm.
5. Place it in the drawer before the dome goes back on.