Have you ever used a "D-shaped" bag before? If so, why would you want one? Everything slides to the center of the bottom of the bag. Unless you intend to sew in lots of pockets, and loops to hold tools off the bottom, its a royal PITA to find something in those pouches.
I watched several members of my gun club struggle to find stuff, to the point they handed me their gun to hold so they could use two hands to dig down into the bags. I decided before I got my first rifle that I didn't want that kind of bag. I do have a rounded bottom bag from October Country, and another that is square. My third one is from heavy skirting leather and is intended to be a game bag as well as a field bag. The jury is still out on that one. Its way too large, and too heavy for what I am used to carrying. But, it was a gift, and I am trying to give it a fair testing before I retire it. For some kind of upland game hunting it may be okay. Like Quail or grouse hunting, where you don't walk for miles. For the kind of open field pheasant hunting I have around here, I am not so sure I want to lug that bag around that much.
I have been going in a reverse direction as far as the size and shape of bags are concerned. I have a small belt bag, and I am trying to limit what I carry so that is the only bag I need. I know its going to be what I use squirrel hunting with my .36 rifle. And, if get my 20 ga. fowler working well with only OS cards, it may be all I carry with my fowler hunting birds.