Encrypt a string to match length of original string

I need a simple c# algorithm that will take a string of x characters and 'encrypt' it to another string of also x characters. It doesn't have to be secure, but it shouldn't be possible to reconstruct the original string by simply looking at the the encrypted string. For instance, if I enter "hello", I should get something like "x=w3q". It shouldn't simply map each character to something else, but it doesn't have to be much more complicated than that. It needs to be symmetrical, so I need to be able to construct "hello" from "x=w3q".

So far I've tried RijndaelManaged and RSA encryption, but the encrypted string is a lot longer than the original string.

Any ideas?