Making your own containers is entirely possible and redwood, cypress and cedar are quite rot resistant although they will not last forever. I would, though, offer some cautions.

Rhododendrons are shallow rooted, but you need depth in a container to ensure drainage. I would not make any container less than 16" or so deep.

A good depth of gravel would be much better than pavers. You want water to drain far away from the planting mix. Pooling on top of the pavers - unless they are porous - would be an invitation to water molds and other root rotting organisms.

Everyone is tempted to provide enough space and planting mix to accommodate the adult plant. Don't do it. The roots of the young rhododendron will be surrounded by a mass of wet or at least moist planting mix that it cannot use. Organic material will be busily decaying, creating a mix that no longer drains well at all. As a general rule of thumb, there should be no more than 1-2" inches of space around the rim of a pot holding a rhododendron. This does require replanting every 2 or 3 years, but the planting mix itself needs replacement on pretty much the same schedule.

I'm not at all familiar with what corner wedges are, but I gather you want to build a large box on top of the pavers. I think you would have much greater success using individual containers placed on gravel or another surface that would shed water immediately. There are commercially available containers that have better proportions for rhododendrons than the nursery can types. I have been using Panterra pots for some years. They have wide mouths and pleasing proportions and come in sizes up to 12" possibly 14" across. Of course, you can make boxes of size you want. It would also be a good idea to contact gardengal. She is a very knowledgable container grower who would be helpful.

I guess my first question would be is there an overriding reason why you must grow rhodies in containers? While I do garden extensively in containers, rhododendrons are not my first choice as a container subject :-) In most cases, they will be much happier in the ground.

If you simply must resort to containers, I am not convinced that size and shape have much to do with your success :-) With a heavily bark based mix - what rhodies and most other woodies prefer - drainage should be sharp so the size of the container is less important. You will not be maintaining a large mass of wet soil around the roots. But I do tend size proportionally for the size of the plant and pot up as necessary. A 1G plant just looks silly in too large a container!!

A 24" cedar planter box is a common presentation here for a good sized rhodie and can last for 10-15 years before starting to rot and break down. But I have also used a standard large black plastic nursery can as well as terra cotta and glazed ceramic. They are all deeper than what you would think a rhododendron's root system would require but the limitations of the container force roots down deeper. It works, believe me :-)

If you keep any woody plant in a container for longer than a couple of years you will need to repot, root prune and refresh or replace the potting soil on a routine basis....rhodies are no exception.

My first concerns are that you are using a good, fast draining bark based potting mix. The exact shape/size of the container is of much lesser importance, although I would not dismiss any of mainegrower's comments with regards to what the pot is sitting on or the size when starting out.

@akamainegrower here are the corner kits that Home Depot sells to make do it yourself planter boxes.

@gardengal48, so I am very big on the gritty mix, and I think I have learned now what a fast-draining soil looks like. I will plant either in pure redwood bark or in possibly a bark-turface combination. Believe it or not, I have experimented with several rhododendrons in bark+turface and they love this combination.

It would help this discussion if we used a specific example. My concern is that an adult rhododendron that will span four feet will have a root system that grows with a diameter of about three to four feet. There is simply no container I could find that would accommodate such a root system. So if you want to raise the plant above the native clay soil, your options are a garden bed or a planter box. In my situation, I thought a planter box might work better.

My native soil is clay, and I do not think I will get optimal drainage with a planter bed. I am using the planter box together with some kind of well-drained base to respect the fact that the rhododendrons want well-drained soil and to allow the soil I am using to drain as it was designed to drain.

Don't worry about the root spread if you want to grow in a container :-) I grow trees in containers that have a much wider root spread than any rhody would, if they were planted in the ground...........the roots adjust to the space they have to expand. What roots are unable to grow laterally will grow downward.