don’t waste your time, just buy one. Mine are from WeldWide(http://weldwidesolutions.com/products_inflatable_purge_dams.html). I have a 2" modified to work in 1.5" to 2", and a 4" modified to work in 2.25 to 4".
For short pipes, just use taperd rubber stoppers in the pipe ends. Easy to get at any plumbing store. Drill 1/4" hole in one end, run airbrake tubing to argon tank and reg, ~5cfm or so pending how many joints there are(more joints, more cfm leakage)