Create custom widget within qt designer to use inside a mainwindow

Question

I want to add a custom widget I have created in qt designer into my mainwindow. Preferably both mainwindow and widget can be created in qt designer and used as is (.ui file)

Because I couldn't get the widget.ui file working, I tried converting it to python but couldn't get it implemented without various bugs (argument errors, what to use as promoted class name / header file, etc.).

Unfortunately every other guide I found online for creating custom widgets has different structure to the one pyuic5 produces:

pyuic5 result:

class Ui_Form(object):

    def setupUi(self, Form):
        Form.setObjectName("Form")
        Form.resize(400, 300)

        # Rest of design...

online guide result (pure python):

class PurePythonWidget(QtWidgets.QWidget):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.layout = QtWidgets.QGridLayout()
        self.setLayout(self.layout)

        # Rest of design...

How can I adapt my code to preferably work with a .ui file or a pyuic5 file. Because almost every approach is slightly different please provide the entire sourcecode too.

In the final product I wan't to use this to fill a Scroll Area with these widgets containing different data. A better approach may resolve this question in a different way.