I bought my rims and disc brakes from Dale. I called him about some question I had on the disc brakes and while talking with him asked him about these inserts. What he said to do was just what #6 in the linked article says, buy some conduit, cut it to length, shape into an oval and place them through the frame. Finish them off as best you can and you’re done.

I’m getting ready to have my exhaust made & fit and will probably install the inserts. Bruce