Sheet metal fabrication can be streamlined and time-wasting mistakes can be minimized by using a cardboard template. Phillip Thomas has some tips.
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When curves are involved, some applications lend themselves to the "layers of masking tape (or other tape) method. Overlap the tape by 1/3 to half and even go across the other direction if you want it stiffer. You can marker reference lines on it and/or outline the hole the piece if filling.
Pull off the tape, flatten it out. Cut the piece from flat stock and work the curve you need into it.
Tape + single-ply cardboard is good too.
Great job on your rock guard by the way.