So I think the second “alpha over” node like showed in root comment would work. The top link in the node sets the size of the working area so plugging in something in the size of the render region will work. (The scale node is obscured due to my carelessness)
it’s still padding but not a bilion of pixels as you feared.
It’s because you are a vampire and mirrors are no longer made out of silver. Just good old aluminium.