Faux concrete numbers might be an idea as an alternative to the real thing.
If you intend to glue the character to a wall and then masonry paint over it a styrofoam or polystyrene number might be sufficient for the purpose.
We could give the characters a thin layer of gray stone coating to get you started. We have been producing faux concrete signs for shop displays for a number of years, even though the signs are designed for inside use all the materials we use are suitable for outside use.
Makes you think, I just might make a 600mm high house number in Styrofoam and coat it, put it on the internet and see what happens.