Hi Pau,
What’s the final use for the rubber stamp you are making? Is it a short use stamp or are you planning on using it a lot? It might inform the sort of rubber you get and the way it is made. If you want something that is getting alot of use then there might be certain considerations to make. I’m guessing you will be using something like a rolling press for something around 20cm square? I can give you a few recommendations on material for things like Lino cuts but completely depends you your final use.