This would be to put on a 12x18 sheet cake.
Do you want it to stand upright on the cake?
If I were you, I'd be more inclined to use gumpaste rather than fondant. Or, if using fondant, I'd add tylose to stiffen it.
I made a stand up, cut-out card using gumpaste. I cut a rectangle out of cardboard, double the length of the desired card. I folded it in half and added a strip of scotch tape to hold it at the desired angle. With the gumpaste rolled to the same dimensions, I carefully laid it over the cardboard to dry. I made several to allow for breakage.
I also made a couple of flat pieces to practice my lettering on which I painted with paste colors, diluted with vodka.
I hope this is what you are looking for. Here is the pic of what I'm describing:
Gumpaste would work really well or do a 50/50 with fondant and gumpaste.