it is a little more than just tube . there is a kick to it and it is bent to match the original frame . you'll find most all the quality crossmembers are 399 . they pretty much keep their prices in line with each other . the cheaper ones are of less quality and usually thinner plate welded together.
you can definitely make your own frame stub. i've done it many times . the reason i like fatman is it is 5/16 wall tubing instead of 3/16 . the one in the picture above was a name brand but it cracked up where the lower aframe bolted. it was the 4th one of them i have replaced over the years. if i were to use any other i would beef up the area where the lower aframe bolts .