You could use the OnResize interaction of the dynamic panel itself.
Say you want 10 pixels between the right edge of the “x” and the right edge of the dynamic panel. The following OnResize interaction of the dynamic panel would work:
Move (widget x) to [[This.width - Target.width - 10]]
For an explanation of This and Target, check out this post. But basically This represents the dynamic panel, Target represents the “x”. So if the “x” is 30 wide, it will be moved 40 pixels from the right edge of the dynamic panel, leaving a 10px space between its right edge and the right edge of the DP.
[Edit] For setting the y position, I’m pretty sure you can leave it blank and the y position will remain unchanged. If not, set y to [[Target.y]], which means the widget’s current y position.