From what I have seen on grammar specialized websites das and dies are not declined when they are used as subjects in combination with sein. Example: Dies sind Zeitungen.
Apart from this, are there any other instances where das/dies can't be declined?
For example when the object of the action is a masculine noun and we wish to replace it with das/dies:
Ich habe einen Hammer gefunden.
=》Ich habe das/dies in der Garage gefunden.
=》Ich habe den/diesen in der Garage gefunden.
Are diesen/den in accordance with the grammar rules here? Do you think das/dies would fit better?
Edit: I would much apreciate some other examples of sentences in which the pronouns das and dies are not declined and don't act as subjects, if there are any.