[Zubersoft]

As Geoff said:

1. Tap the shape icon (which is a line to start), then tap it again to display the settings
   
2. Switch to the rectangle tool and set the line and fill color to white
   
3. Tap the favorites icon (a star) at the top left and tap the + icon to add the rectangle as one of your favorites. This lets you quickly switch to drawing a white box whenever you need it
   

Mike