As a costumer I was asked to research this garment. I found the chief problem to be the vast disparity between the duster Hellboy wears in the comics and the coat worn in the films.
There is also a difference between the coats worn in the two films. In the first film the coat seams to be almost entirely made of leather, while in the second film the coat appears to be mostly canvas with a wide corduroy collar and only a bit of leather in the trims.
In the comics the coat is drawn as if simply made of canvas. In light of the speed at which they must be replaced this only makes sense.
Both in the comics and in the films the best match for the overall garment seems to be the “Australian Drover’s” coat. (Many patterns for this style coat are available but Folkwear’s seems to be the best.) The greatest difference between the comics and the films is that the coat in the movie retains the coat’s shoulder cape, which is never seen in the comics.
My client wanted a garment that was more comics than movies, so I eliminated the cape, but the buckles on the right sleeve were retained from the film coat. (I also made the coat asymmetrical in scale as well, enlarging the right sleeve by 175%, allowing HB to remove it, drawing his hand through the now much larger arms-eye at the right shoulder. This puts a wide box pleat behind the buckles, and nicely justifies their placement.)
My version is made of pale tobacco tan suede cloth trimmed with quarter inch dark brown corduroy, The buckles are antique brass, and the buttons are inch and 1/8th horn. Also both sleeves can be rolled up, (like a safari shirt) and buttoned in place with straps. (In the comics HB is often shown with one or both sleeves rolled up, the addition of the straps was both utilitarian and matched the rest of the changes made to realize the garment.)
One final note; The red and white clover leaf patch sometimes seen on HB’s shoulder alongside the BPRD emblem is the insignia of the U.S. 4th Army. (Undoubtedly where HB received his combat training. His relationship with the 4th Army is probably no coincidence, HB grew up on a government base full of scientists doing “secret” work, and the 4th Army was assigned the duty of guarding Los Alamos during WWII.)