I'm so sorry you are going through such pain. The plaster imprint sounds like a good idea, but I would like to offer another, in case it might help. If you have some pics of Sid that you like, maybe making a collection of them in an album, or maybe a poster with the birthdate and details would help you honor his little life.
Wishing you peace to help with the sorrow.
Wishing you peace to help with the sorrow.