I also started with a cardboard template, then I made one out of thin aluminum and after I was happy with the fit, I gave the aluminum model to a guy to make me one out of steel. That way I knew it would fit OK. I put some round holes on the bottom so I could get a ground connection on the frame rail for my relays and also to drain any water that entered the box from the front. My relays and fuse panel (and auto-reset circuit breakers for my headlights)are inside the air box as well (last photo)
Jim
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