Answer: Unfortunately, I am not aware of any plant growing to quite those dimensions -- 20 to 40 foot puts you squarely into tree territory. That would be a structurally difficult plant with so much height and so little width.
The SkyRocket juniper is very narrow indeed, and although it does reach some height, does not grow as tall as into that range. The juniper Prairie Pillar approaches 20 feet and is quite narrow as well. A slightly wider juniper reaching about three feet wide and 15 tall would be Spartan; Blue Arrow is in the same height range and only about two feet wide. These are all evergreen from the ground on up and would not require trimming. They require full sun and a well drained soil.
I am not sure why you think the arborvitae need a lot of care, they are typically considered fairly low maintenance plants when left to reach their natural size and shape -- and when planted in a location and at appropriate spacing between them where no trimming is required. There are varieties in the four foot wide range. These plants also do best in full sun and do well in average to better soil. They are not quite as tolerant of a dry location as the junipers and some varieties will turn bronzy in the winter.
If you are trying to plant along a property line to screen a neighboring building or block a vista type of long view, you will need a lot of plants and they will need to be tall. It is sometimes easier to set your plants closer to your viewpoint instead. For example, plant your shrubs along the patio you want to screen or in an island planting bed closer to the window you want to block. This works the same way as looking through your hand when it is held close to your face as opposed to looking through your hand when your arm is outstretched. It blocks more of your view the closer it is to you. So sometimes a strategically located planting can do the job more effectively and sooner than waiting for a tall hedge to develop.
Sometimes, too, it is effective to place an awning or trelliswork or vine covered arbor to shield a window or patio to avoid being overlooked, or to use a large deciduous tree as a visual foil and overhead protection -- although it does not fully block the view year round, it provides a psychological sense of separation especially during the summer months when you would normally be outside. Also, when space is at a premium, a tree has a small footprint, just its trunk, while the branches can reach far and wide.
I hope this gives you some ideas. Your local nursery may also be able to suggest some plants based on a more detailed knowledge of the site and what you are trying to accomplish -- and your budget and time frame. Good luck with your planting!
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