Passive solar water heating depends on the fact that hot water (and air) rises in a system. So you need a complete circuit containing a solar panel and a container tank joined by piping. The tank needs to be ABOVE the solar panel. The water in the solar panel gets heated and it naturally rises. It flows into the tank above and cold water from the tank flows down to take the place of the warm water.
Your solar panel can be as low tech as an old radiator panel painted black and facing the sun.
Yes, you can build your own from scrap materials. One tank, one old radiator panel, some flexible piping and joining clips / materials makes a simple, easy solar water system.. You could put the solar panel on the wall of your house (facing south) and put the piping up into the bathroom, to create a heated towel rail.
This simple system, however, needs to be drained in winter, if you have a cold winter, because it can burst in the frost.
There are plenty of DIY solar systems available, including photovoltaic panels which use the sun's energy directly. You can also build one of these, collecting energy in old car batteries, or again, heating water in a tank.