For the bright red color, I'm adding in some miniature carnations (also known as spray carnations). There are usually anywhere from 3 to 5 flower heads to each stem, so you can get a lot of mileage out of a single grower's bunch of carnations.
This is where the additional needed height is easily obtained by using the carnations.
Alternate around the design, not being too exact in the spacing. You want your design to look natural and garden grown - not too "perfect" and precise with flower placements.
Just so you have a good overall cover and mixture of both red and white in the final flower design.
You could have easily made this much larger by using standard carnations and lengthening the stems of the white chrysanthemums.