I use my milling maching to slot the top. If you don't have a mill, I'm sure you could build a simple fixture that would allow you to use the cross slide on your lathe to do this. Notice that I have a vice stop clamped in place. It both "backs up" the piece and makes it a simple matter to index the piece from the sides. Also notice how close the slot saw shaft is to the cap. The only critical thing here is to keep the distance between the tops of the slots bigger than 0.250. This is just under the 0.281 ID of the tube. DO NOT slice the top all the way through the center. The top of the cap deflects the gas mixture down and around the tube top.