There are numerous common concise ways to write the 18th century, at the risk of needing the right context to be understood, including “C18th”, “18c.”, or even “XVIII” by itself.
These are even more impractical, so I wonder what your point is? I can come up with an even shorter way to say 18th century, by using base26 for example, so let's denote it as "cR". What has been gained?