I remember watching a Kiwi surfcasting program where they were using 2 sinkers designs that tend to stay put.
One had flat surfaces pyramid sinkers so that the flat surfaces sat on the bed as opposed to round surfaces which would assist the roll. See picture below, however weight was at the other end and the pyramid base was not base as in this picture was not flat but tapered for better aerodynamics when casting to improve distance.
The other one had wires incorporated into it (Breakout sinkers) which dug into the sand and got a firm hold there, the wires were fitted into groves and acted like clips, when you retrieved your sinker back the clip came loosed and flipped backwards and the sinker comes free, all you need to do is re-align or load the Sinker before the next cast. This sinker really holds the bottom, there are several types available, see images below.
Perhaps you know about these sinkers already and they are not quite made of sand, but as you asked about shape...