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If you use curved sides on the boattail it will match the curved front. The rack could have an expanded metal mesh floor for the firewood. Would the boattail be hinged like a truck lid so you have access?
When you open up the metal duct, does it have a standing seam on one edge?
I would think the thing to do would be to open it up and flange one edge out, maybe 3/4" to 1" to fit and seal against the curved face, letting the other edge lap onto the outer wall. I'm not sure which edge would be better to flange, or you could snip the standing seam off.
The outer edge may need slits cut to relieve the metal, it depends. There exist crimping tools that you can work along the edge to shrink it. What does the existing edge look like? An aluminum extrusion?
At the top, to terminate the edging, you could cut a lengthways gore in the middle of the curve and overlap and rivet/braze it to make a[n outward] tapering tail. It needn't add frontal area, but right there—it may not make any difference. I'd taper it out at the end of the front face for rainwater run-off.
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