I make mine from 1/4" plate.  I make a template from cardboard in the correct shape and size and trace the cardboard and start cutting/ grindiing.  Make sure you sleeve the frame for the bolts too.  I drill the outside of the frame big enough for the sleeves to fit in and leave them about 1/16" long so they stick out just enough for the plats to not rest directly to the frame and I put everything together and snug it down.  Then I put short beads on the plates and pull it back apart for final welding.  Id also plate the inside of the frame too.