Convert the lines to shapes (but not the half circles which are already shapes). Then will all the lines and half circles selected, Arrange > Combine Shapes... Add Shapes. If you add an outline and white fill your shape should look like the above.
NOTE: If you see lines inside some of the long horizontal and vertical rectangular shapes it is because you did not overlap the ends.
Rotate the shape 45 degrees. Make a clone, and flop it horizontally and vertically using the Flip Horizontal and Flip Vertical icons on the Infobar. Align the two shapes as shown.
Select both shapes and Arrange > Combine Shapes... Add Shapes.
