```typescript
export function layConveyor(floor: string[], row: number, col: number, length: number): string[] {
  if (row < 0 || row >= floor.length) {
    throw new Error("Row is out of bounds");
  }
  if (col < 0 || col + length > floor[row].length) {
    throw new Error("Run would leave the plan");
  }
  for (let i = 0; i < length; i++) {
    if (floor[row][col + i] !== '.') {
      throw new Error("Cell is not open");
    }
  }
  return floor.map((row, r) => (r === row ? row.replace('.', '=', col, col + length) : row));
}
```