Make another duplicate of the R, align it as shown on the left, and Combine Shapes > Subtract Shapes. This removes the shape of the R from the combined shape. Select the shape and from the Arrange menu, select Break Shapes . Select the two inside shapes and delete them.

Before you do this, let the R in front extend just a hair to the left and down, otherwise some of the underlying shape may be left.

OK If you happened to make that diagonal line and didn't delete you can use it for this next operation.

Create a diagonal line that passes through the shape as shown on the left. Select the line and the shape and from the Arrange menu, select Combine Shapes > Subtract Shapes. With the shape selected, select Break Shapes from the Arrange menu. This creates two distinct shapes as you can see. Place the original R in front so it aligns as shown.

Finally create a small shape (shown in pink) for the bottom of the opening in the R. Bring the original R to the front (Ctrl F).