Use shorter fronds to fill in the top and sides of the bouquet holder.
Keeping the top pieces short will allow you to maintain your curved shape.
The outer pieces of leather leaf should angle upwards slightly when inserting it into the bouquet holder on the sides.
As you move towards the middle of the holder, the pieces begin to angle upward, until you reach the middle and the leather will stick straight out.
This gives dimension and depth to your bouquet, rather than a flat look from the sides.
Don't pack the greenery in too tightly at this point. You still have a lot of other greenery and flower stems to insert into that foam.