UseSprite
for border and attach it as a child of Camera2D
Update
Viewport version would involve few steps:
1.Creating main scene containing "border", Viewport, and Sprite
2. Creating level scene - actual game that will be played
3. Attaching level.tscn
as viewport
child
4. setting rendertarget.vflip on viewport
5. Attaching into sprite
texture new ViewPortTexture
6.setting appropriate dimension of sprite
and viewport
here demo of above in my godot-recipes
project