if you look closely you can notice that most of the light on the render is coming just slightly from the right of above the render so you should have placed it there and lowered the opacity of it to like 15%. the background looks like you just used the cloud filter with black and red and smudged it a little and the only real blending is where you smudged the render and background together at the bottom. Try adding some c4d's, gradient maps, brushes etc., also remove the 1px stroke and go with maybe a 3px black border for just the top and bottom.