An arched hedge entryway for the roof garden

Last week while studying the roof garden, I decided that what the right side needs is an evergreen hedge with an arched entryway cut into it:

I'm craving more of a feeling of privacy on the side of the garden, so if the tall evergreen hedge doesn't do it, I might even add a door:

The hedge would run along the edge of the patio, perpendicular to the new boxwood hedge I just planted.  It's a really short span, so I might need as few as 5 plants. 

I did some research to find the best type of evergreen shrub for the job.  First of all, I had to take into account the tiny size of the garden, the lack of much soil depth for roots, the weight load restrictions and the fact that the garden is in Zone 6.  Ideally, the shrub would grow quickly to about 8 feet and max out at a depth of about 2 feet.  I like the look of Italian cypress, but of course it's too cold to survive here.  I did decide that something dark green and glossy would be best. 

Right now I'm looking at 'Sky Pencil' holly:

'Graham Blandy' boxwood:

And 'DeGroot's Spire' arborvitae:

It's pretty confusing because the height that each is supposed to grow to changes depending on who you ask.  For my purpose, the difference between 8 feet and 15 feet is huge.  I want a lush, robust-looking hedge, but I don't want a giant that's going to get out of control.  Also, I want something fast growing to its mature height because I'm impatient.  I'm going to pay a visit to my local nursery and see what they've got.