Allow Many QWidgets to be Shown into One QWidget ?

[Fatla]

I've created Many (.UI) , made a (Header File + .cpp) to each .UI in order to implement their own slots , & Linked them by making a widget to be shown using :

// CustomSlot2 implements the slots inside the 2nd .UI
CustomSlot2* widget = new CustomSlot2();   
 widget->setAttribute(Qt::WA_DeleteOnClose);
  widget->show();

// CustomSlot2 implements the slots inside the 2nd .UI
CustomSlot2* widget = new CustomSlot2();   
 widget->setAttribute(Qt::WA_DeleteOnClose);
  widget->show();

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CustomSlot2 Constructor's Code is :

CustomSlot2::CustomSlot2(QWidget *parent): QWidget(parent)
 {
    formTwo.setupUi(this);
 
 }

CustomSlot2::CustomSlot2(QWidget *parent): QWidget(parent)
 {
    formTwo.setupUi(this);

 }

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So far so Good . However , by making this approach there's a flicker when you Open & Hide/Close Windows .
I've thought in an alternative way ;that I'll Drag & Drop a QWidget into my Main Window using QT Design.
The Problem : How Could I open my .UIs into the QWidget that I've added @ MainWindow .

The MainWindow's header file generated by "make" :

class Ui_MainForm
{
public:
    QPushButton *btnProcess;
    QLineEdit *txtInput;
    QLineEdit *txtResult;
    QPushButton *btnNext;
    QWidget *widget;
//........................ etc

class Ui_MainForm
{
public:
    QPushButton *btnProcess;
    QLineEdit *txtInput;
    QLineEdit *txtResult;
    QPushButton *btnNext;
    QWidget *widget;
//........................ etc

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Thanks .