Ran into this issue and ended up sorting it out in the end, posting my solution in case anyone else still needs help:
On your UMG, add an image that fits to the size of your screen.
Then, create a material and assign it to the new Image’s brush.
Inside that material, make sure Material Domain is set to “User Interface”, Blend Mode is “Translucent”, then set up like pictured below.
The result should have the scope nicely scale to match screen size without ever affecting the original image’s aspect ratio.
