How do I access data in "int20_t"?

I got a lot of memory pieces in 512 bits. I need to write numbers in 20 bits.

For example:

// a piece of 512 bits memory
void *p = malloc(64);

struct Coords {
  int20_t x, y;
};

// usage
Coords *pcoord = (Coords *)p;

for (int i = 0; i < 512 / 40/*sizeof(Coords)*/; ++i) {
  pcoord[i].x = i;
  pcoord[i].y = 2 * i;
}

How to implement int20_t ?

I need exactly 20 bits for an integer. The numbers must be continuous. ( [0] to [19] for the first Coords.x; [20] to [39] for the first Coords.y; ... There are 12 pairs of Coords with 480 bits and 32 bits for the pad.)