The main ingredients of my potting soil mix: red lava rock, potting soil, and builders sand.
The builders sand has gravel up to about 1/2" in length and down to 1/16" or slightly under. The mix ratio was one bag of lava, one bag of builders sand and one bag of potting soil for a volume ratio of 1:1:2. Not shown is the Osmocote slow release fertilizer that I added in lesser amounts than I thought the directions called for.
Here's what the mix looked like on the blade of my spade - the gravel and lava are not as apparent as one might think given that they make up 50% of the mix.
The pot that I had to fill seemed immense. I don't know if I'll regret it, but I used four empty plastic bottles (3 liter bottles and a 1.5 liter bottle) to take up space in the bottom third or bottom quarter of the container. I placed a layer of soil mix into the pot first to give it some weight in the bottom, then filled it up progressively with the empty bottles laid two on top of the other. These are the upper most two. Even with the bottles, the total volume I had to fill took about 3 cu. ft. of potting soil mix combined with the appropriate amount of lava rock and sand.
The photo below gives an idea of the size of the container and its site - on a raised area near the front door entry. I used wooden wedges to stabilize the saucer that the pot sits in, since the tile wasn't perfectly flat. The two smaller plastic bottles shown here I did not use. The pot is from the "Pot Depot" aka "Pot Mecca" aka Pottery Manufacturing and Distribution in Gardena where they have great deals on factory seconds and even have a teacher discount (advertised on their YouTube link, but not advertised in store, so I didn't know to take advantage of it). $110 got me a ~36" tall glazed pot and saucer.
Here's the final result. That's Giant Chain Fern (Woodwardia fimbriata) and Yerba Buena (Satureja douglasii). The fern should get several feet tall and the Yerba Buena should trail over the edge of the pot nicely. Let's hope it works!