And of course when I make a "replace content" the size differs from the original document and doesn't match anymore.
Hi
Try this:
Make a layer and fill it with any colour. Make that your smart object - the size will match.
Now open the SO and add your render as a new layer and turn off the filled layer
Save the psb and it will be in the same position
Alternatively you could use a linked SO :
Make a new document and fill it with colour and save it as doc A.psd
Make a second document (doc B.psd)
Use File > Place linked to place doc A as a linked smart object in doc B
If you now add your render into doc A and save it - doc B will be updated when you open click on the update link in the layers panel
Dave