For the problem described pycrypto is the best solution. Blowfish is simple and secure. The method you want to use is called "security by obscurity". But chances are very high that the "homebrewed" scheme you will invent will not stand any serious crytoatack.
First of all: both sides (sender and receiver) have to agree on the session key used. And this is the most "dangerous" exchange. Sure you can hard coded the key on both sides, but periodically you have to change them. I was facing similar problem: had to secure legacy Inet server. I started with stunnel (worked great, but was a little bit sluggish). Then I figured out how to implement IPSEC (both sides were W2K) and this was the safiest solution. Sorry but it does not involve Python :( -- http://mail.python.org/mailman/listinfo/python-list
