custom type similar to varchar(#)

I think that this question was adressed before, but I looked around and
couldn't find the answer.

I would like to try to emulate the behavior of the old style varchar(#), in
which it automatically truncated values that were oversized. To do this, it
occured to me to try making a custom type, but for that I would need to
accept the numerical argument as part of the type. I could not find any way
in the documentation to create such a type. It would occur to me that I could
try to use the atttypmod value as varchar appearently does, but I'm not sure
exactly how to do that.

Any suggestions?

Thanks,
    Jeff