I believe the probem is in the pulling of the wood. It's real easy to pull off an a slight angle since you're facing the saw. Why not just push it all the way through with a push stick? That's what I do. The featherboards will do the work of keeping it along the fence. If your short or temporary fence is just past the center of the blade, It has to cut it correctly. But if you pull it on an angle, you have so much torque that you can over-ride the force of the featherboard and get an uneven rip.

Been there, done it. Before you go nuts with your set up, try pushing it all the way through. I use a long push stick for that and not the little store-bought short thing.

BTW: I picked up the shorter fence thing form "The New Yankee Workshop -Norm Abrams" on TV. He uses it all the time for board ripping. The long fence is good for plywood etc. where you know you have a straight edge or for shorter wood. Even if you use a long fence, pulling the wood can knock the piece out of line and you have to be cautious. When you're behind the board, you kind of sight down it as you are pushing and actually you push with force toward the fence.