Try to build from ground cover (towards the front) up in height as you move towards the back of the bed. So if you don't have any grasses with height look at Feather Reed or Zebra grass, or similar. They have a fairly compact growth and will give you necessary height in the bed design. And don't forget your mulch and weed barrier to keep it tidy.

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If you are concerned about height at the back (I wouldn't be as the variety of height adds texture, interest and drama - something this garden bed needs desperately) place the KF grass towards the point at the end pictured. I would also prune back the round shrub (when it's dormant) or consider moving it. It's already overwhelming the space, and goes against the "modern" garden look you mentioned wanting.

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