Place an instance of the light component away from everything else.
Select it > Context-menu > Change Axes
Pick a point on the 'frame where you’d like it to overlay the surface it’s going to be glued to.
Make X red the long direction, and Y green the short dim, but note this step carefully - set the Z blue axis to be pointing away from the surface onto which it will be placed.
Confirm and delete the temporary instance.
Also delete the current instance as it needs a new version adding…
Now go into the Components Browser > Model pane, and select the light component, Edit it and in the Alignment options choose Glue to: “All” and tick the ‘Cut Opening’ checkbox.
Place an instance of it onto any face - including your sloping soffit - and it should now glue to the face, I chose to ‘cut-opening’ because it looks like it’s a ‘box’ recessed into the face it’s placed on to.
If for some reason it doesn’t seem to glue as expected, first check that its axes are correct if not correct them and place a new instance; next check that the face you are gluing to is not ‘reversed’ - View > Monochrome to see this - if your Style has a distinctive back-face color - default is usually a blue-gray - you should not see it - because a gluing object always matches its face’s ‘normal’ an incorrectly oriented face will get the instance ‘back-to-front’. To fix a backwards face, Select it and use the Context-Menu to ‘Reverse’ it…