> I've looked for open source/open hardware printers some time ago

You want a pen plotter. Or its cheaper, more expensive cousin, the Axidraw.

> As you mentioned fire-resistance, do you also use some sort of fire resistant paper? Because I don't think anything in made of paper would survive an actual fire.

Actually there are document safes with built-in insulation which will keep regular paper documents safe for time X under fire conditions Y (it's some EU standard classification). Certain ways of mounting safes makes them intrinsically fire-safe(r) as well.