The idea I had would be to use a 10" piece of 5/8" allthread rod (the
kind normally found at Home Depot or Lowe's) and two 1/4" mild steel
plates with a center hole drilled slightly larger than 5/8".


Don't go there. That grade of all-thread is not very strong and you are
taking a big risk.

If you don't like you store bought ones, buy better ones.

John