Looking at the the caQtDM repository does not reveal any Qt custom layout manager. Instead, the application likely responds to resize events of the windows directly.

To co-exist with use of the standard Qt layout managers, a custom layout manager seems the right way to proceed.

A basic version of the new custom layout (name to be adjusted per agreement):

class MedmLayout(QLayout):
    """
    Layout Manager for MEDM widgets.

    Widgets are added to this layout with absolute coordinates (x,y,height,width).
    When the layout is resized, the manager will resize each of the
    widgets in the layout.
    """

shows up in designer by following the patterns in qtplugins.py:

MedmLayoutPlugin = qtplugin_factory(MedmLayout,
                                    group='PyDM Layouts',
                                    is_container=True,
                                    extensions=BASE_EXTENSIONS)

but there are problems:

(dev-pydm) zorinvm@zorin22:~/.../slaclab/pydm$ designer
Loading PyDM Widgets
Exception occurred while running Qt Designer.
Traceback (most recent call last):
  File "/home/zorinvm/Documents/projects/slaclab/pydm/pydm/widgets/qtplugin_base.py", line 119, in createWidget
    w = self.cls(parent=parent)
TypeError: 'parent' is an unknown keyword argument

Designer: The custom widget factory registered for widgets of class MedmLayout returned 0.
** WARNING Factory failed to create  "MedmLayout"

https://stackoverflow.com/questions/5659875/custom-layout-in-qt-designer

Advice is to write a custom container (QWidget, QFrame, ...) which can be interfaced as a designer plugin.
Then call the custom layout from the custom container.

Now called AbsoluteGeometryLayout

QFrame is a subclass of QWidget

The additional attributes provided by QFrame are not expected. The AbsoluteGeometryWidget will subclass from QWidget and call AbsoluteGeometryLayout which will have zeroes for all the margins by default:

Drat!. Even with the custom layout called from the custom widget, the custom widget is not designable in the sense that child widgets can be dragged into it. Unless the window is based on the custom widget. In either case, widgets added will not be part of the custom layout.

Rethink it

A QWidget can have widget children. It can manipulate the geometry of each child during a resizeEvent(). One way for that to be successful is to keep the original widget size and then compute the horizontal and vertical scale factors with each new resizeEvent.