Here is what I have done, borrowing very heavily from another person's design:
I took 6' green steel fence posts (presumably you could use any length -- i.e. smaller), and cut 20" or so lengths of 2-1/2" PVC (also could use 2" if preferred).
Using 2 galvanized nuts and bolts, I mounted the pvc on the top portion of the fence post, drilling holes in the PVC, etc.
Advantage: very sturdy, heavy duty rod holders, once you get'em in they are in until you pull them out. relatively low cost.
Disadvantage: quite heavy -- much heavier than more expensive aluminum ones I have seen.
hope this helps.
I like mine a lot -- the one thing I'd like to do is somehow round off the top of the PVC so it does not dig into the cork tape on my rods -- any ideas for that would be greatly appreciated.
CS
btw, on most areas on the beach I have to use a shovel to get them in.