I thought of one you could do very simply, but I don't know if it'd work...
If you had a top case fan which was rather powerful, make a little tube with card board, and place the cardboard covering the CPU fan, and then to the top of the fan, so that the air goes straight up and out.
It'd probably heat things up more though, since its more enclosed, but I haven't tested it... maybe putting small holes in it would help