From qtnode

Adding more widgets to Qt Designer

To add new widgets to Qt Designer, edit the widgetbox.xml file found in your Designer preferences folder. Under Linux, this is $HOME/.designer; the location may vary with other operating systems. QWidget can be promoted to any other widget in Designer, making it an excellent candidate for addition.

Add this code to widgetbox.xml, before the final </widgetbox> tag:

<category name="Custom" >
    <widget class="QWidget" icon="" type="default" name="Widget" >
        <property name="text" >
            <string>QWidget</string>
        </property>
        <property name="objectName" >
            <string notr="true" >widget</string>
        </property>
	<property name="geometry" >
	    <rect>
                <x>0</x>
                <y>0</y>
                <width>160</width>
                <height>80</height>
            </rect>
	</property>
    </widget>
</category>

If you wish to add additional widgets, add additional <widget> blocks inside the <category> block.

Note: If you don't have a widgetbox.xml file, creating one will override the defaults. Instead, copy the Default Widgetbox File into widgetbox.xml and edit that.