Can I install an outlet on a freestanding pillar platform?
Where I could not go deep enough I ran rigid ~5” below the surface then used a 5 gallon bucket brought up the pipe from the bottom to a bell box went back out to the next one. I  filled the bucket with concrete. That gives more than 1 cu ft of concrete and was allowed by my AHJ. The bell box’s I used had 3 openings 2 on the bottom for in and out and one on top for a light, every other one I put a receptacle on the box. I cut the bottom out of the bucket but did press the bucket into the gravel I had there. If I do it again I would put the top of the plastic bucket down in the gravel not the bottom of the bucket as they are slightly tapered and getting the bucket off was a pain with the taper going the right way it would have been much easier but if you have a few extra 5 gallon buckets this was allowed in my jurisdiction as sufficient I think 1 cu ft of concrete was the minimum.