Adding a drain plug 727

Steel pipe couplers can be difficult to find. One place they exist and you can sometimes get time free is on the ends of 20' lengths of pipe. In this locale, steel couplers are not to be used in has piping so unplated pipe "protectors" are not used

You can get "although usually heavy" steel couplers at a place specializing in industrial/ tractor hydraulics

Yet another option is a steel pipe bushing. You want UN plated, plain steel. You could, to reduce size, turn the bushing down "some" in a lathe, or even chuck it into a pipe nipple "rig" in a drill/ press and grind it down

Sure too bad Ma didn't think a little before "not" installing drains!!!