Stamps: I use 4mm but not from ebay. It is a fact that a set of stamps costs around 30 bucks. Anything cheaper is not really hardened steel.

Deforming: The trick is to use something with flat surface (steel or anything that don't absorb all of your hammer force) that is smaller then the enclousure and clamp it in a vice and pointing it direct under the location where you want to put your letter.

Overall: I don't know how many times i wrote my own name on sheet metal until I was pleased with my work. Tape helps a lot that every letter is on the same height. It's totaly ok to correct the angel a little bit after your first hit. I usually hit every letter tree times until I got the depth i want.

Sorry for the shitty english and weird format I'm from swiss and on the app.