Here is a catch can I made for my car. It is made out of 2.5"x4"x3/16" aluminum tubing and some 1/8 aluminum plate. I used a band saw to cut the tubing to the shape I wanted and then used a press brake to cut the 1/8 plate for the top and side. On the inside, I used some of the aluminum plate and put it on a milling machine to drill the holes. I made two of these and sandwiched in between them is some stainless steel wool to help separate the oil from the air. These plates were tack welded on the inside and then the top was welded on.
All of the welds were done with a Miller Dynasty 200. The amperage was about 135-165 amps depending on the weld. AC frequency was at 90 htz and AC balance was at 70. 3/32 black tungsten with a #6 gas lens at 20 cfm. I also used 3/32 5356 filler rod. Also in the pictures you can see that I cut the plates about 1/16-3/32 of an inch smaller than the actual dimensions in order to get a nice flush weld.