I used rocks successfully on the bottom of my 5-gallon bucket with a 3-inch level drain hole, it's kind of similar to this raise bed video. This is another option, the reservoir is filled once a week, and the nutrients in the soil don't get washed out.
I saw that video sometime last year. It's pretty good.
Here is one that I think you might find is even better. It's 30 minutes long, but he packs in all kinds of information. If you are interested in sub-irrigated planters, it's definitely worth your time to watch and learn.
Using his concept, I made three elevated sub-irrigated 2X4 foot planters for out on my deck. They are by far my best planters because the water reservoir provides a constant source of water to the growing plants.
Here is a picture of one of my elevated sub-irrigated planters full of beans about 3 weeks ago....
There are some significant differences in the water reservoirs. Instead of filling the bottom of the planter bed with rocks, Aldo Pepper uses 4-inch perforated drainpipes with an overflow hole drilled in at 3 inches above the bottom. That leaves a 1-inch air gap in the system, so the roots don't drown. Also, rocks take up a lot of volume in the reservoir, whereas the hollow drainpipes can be filled with much more water in the same amount of space.
I calculated that my 2X4 foot 3 inches deep reservoir holds about 15 gallons of water. I imagine if that space was filled with rocks, you would have a lot less volume of water in the reservoir. The result is that I only have to fill up my water reservoir with drainpipes about once a month in the summertime - providing we have normal rainfall.
We had a terrible hot, drought summer a few years ago and it did not rain from mid-May to sometime in September. That year, I had to fill up the reservoir in my sub-irrigated planter about once a week. But my plants that year looked as good as the picture above that I took a few weeks ago. Meanwhile, my in-ground gardens had all dried up and died. My "normal" raised beds dried out and died. My hĆ¼gelkultur raised beds survived with maybe 50% production. That drought year really showed me the value to my hĆ¼gelkultur raised beds and the sub-irrigated planter.
One improvement I made to my sub-irrigated system was to cut a strip of swimming pool noodle and putting it into the PVC drainpipe to act as a water level indicator. When full, the pool noodle as a water indicator sits about 3 inches above the top of the fill pipe. As the water gets used up, the indicator drops down. When the indicator is level with the drainpipe, I refill the reservoir. Using a yellow pool noodle as a water level indicator makes it easy for me to see if the planter needs to be refilled, or not.
The only negative thing I would say about the sub-irrigated system is that it costs good money to line the planter with heavy duty pond liner like I used. The 4-inch perforated drainpipe also adds to the expense. I live in a snow state, so I did not think a 6-8mm plastic liner would survive the freezing weather outside in the winter. That would have been a lot less expensive.
All my raised beds out in the yard are 16 inches high, 4X4 foot square made out of free pallet wood, and I use the hĆ¼gelkultur method. It costs me only a few bucks for the screws and nails to build each raised bed. If I had a bigger budget, I would like to make raised beds with the sub-irrigated concept. But pond liner is pretty expensive, so I am happy using the hĆ¼gelkultur method instead.
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