Where are you? If you have sandy soils combined with alkaline water, even your raised bed may not meet long term requirements for azaleas...

Rhododendrons and azaleas flourish in soils high in organic content. One drawback of doing a raised bed of highly organic soil is that it's going to settle some over time so plant high to begin with. It's best to use organic material that has already begun the breaking down process rather than fresh plant remains which will use more nitrogen completing their break down....and avoid mushroom compost (sometimes called mushroom manure) - which is not composted mushrooms but the medium in which they were commercially grown, contains lime and is too high in salts.

18" is a recommended depth for raised beds over alkaline or badly draining clay soils, I would assume the same is true for sand. It sounds like you plan to excavate and construct a bed of amended soil, I wonder if it wouldn't work better if you could plan for 18" above grade although I'm not certain that would be as important for sand as for clay base.

I am about eighty miles ssw of Dallas, near Tyler and no alkaline water, just good drinkable, deep well water. I plan to build a box with a bottom in it. Like a large planter setting on the patio. It will be filled with locally composted leaves etc. and top soil. I could use nothing but compost, but that would be a total waste of good compost. When I built my other planters I filled them to the top and soaked them with a slow stream for several hours and then let them dry for a week or so before planting, no settling problems so far and I have about 20 of various sizes fron 2x2 feet up to 4x8 feet. I basically just need to know how deep the roots of azaleas go, so I will not have them growing out the bottom of the planters. I pretty much have my whole garden in raised boxes, with drip irrigation.

Azaleas grow a fine dense network of surface roots that will extend at least as far as the widest branches in the ground, with just a few larger anchoring roots that will grow deeper. For that reason azalea pots are normally wider than deep - starting with the smaller sizes typical is 6"x4" (wide deep), 12"x8" etc, but you get the idea/ratio I'm sure.

My concern about settling had to do with the need for the large volume of organic material in ideal container medium, organic material/compost will continue to break down over time, thus the settling. Pay close attention too to drainage of your medium in a container, it will be especially important in a warmer climate - too moist roots + heat= root rot.

One suggestion for a good long term container mix for rhododendron and azalea is minimum 50% premoistened peat, into which approx 25% leaf mold, 25% sandy loam has been incorporated.