What is the soil like all these roots are growing in? Well endowed with organic matter or severely lacking in nutrients of any kind which wouold cause these shrubs roots to need to really reach out in search of both?
Putting a planter in, with really good soil, will not solve you problem since the roots from those shrubs would simply invade the planter, in search of the nutrients and moisture they lack now and need.
Fix the soil you have so there will be ample nutrients and moisture for both plant species.

Whenever you plant within an established root zones (such as is the case with your junipers) you need to supply the newcomers with more water than you might expect.

As it stands, your attempts were with water guzzlers -- namely Australian tree ferns, brugmansia, hydrangea, azaleas.

You'll have a better chance of success if you choose plants which do well with minimum water.

Oh yes. About that proposed planter box -- it will be "heaven" for the juniper roots!

There was a great discussion about this in the Container Gardening Forum in April. Paul, rocklandguy, came up with a really good solution: house siding. (Thanks, Paul!)Here's a link to the thread. HTH. -ConnieCola

Here is a link that might be useful: Tree Root Barrier