Duster Battery Tray

They might be out of stock but, I recently bought one for my ‘75 Dart Sport and, it is a perfect fit (3 bolt holes lined up perfectly). Got it off Rockauto. Cost about $20 + shipping and is an aftermarket direct fit (not a universal).

Update: it’s now $9.49! Part number: GMK211130067 by Goodmark. I believe this same tray would work for your ‘73 Duster. The Dart Sport and Duster bodies share a lot in common.