Game window of 450 x 800 for portrait or 800 x 450, 960 x 540 for landscape. Those are 16:9 HD tv ratios.
Layout set to same ratio , but much larger size, real HD 1920 x 1080. full screen scale outer setting.
On start of layout scroll to center of layout ( 960,540 ), create a guide sprite ( just a rectangle ) size it to your 450 x 800 or whatever game window size you use and place in the middle ( 960, 540 position ).
Now have all of your content fit that rectangle.
Outside that rectangle fill with background graphic or graphics.
That seems to work for me, with the added benefit that you can play it at full screen on desktop too, esspecially if you are creative with your backgrounds.