I don't know of any custom helmet makers, but Arai helmets come in several shapes, and have numerous liner and cheekpad sizes for each helmet size and shape. A couple of the new models also have adjustable pads, so they can be fine-tuned. If a good Arai helmet fitter cannot fit you, you have a one-in-a-million head.
Scotty's right. Arai makes 3 different head shaped helmets with interior padding in the Vector-2 that actually ends up giving a 4th.
I sell Arai helmets and have yet to find a person that can't fit into one of these head shapes. But I also know that it could happen.
I highly recommend getting to a good dealer that carries the Arai line and try on the Corsair-V, (Intermediate head shape), Profile (Long Oval head shape), RX-Q (Round Oval head shape) and Vector-2 (Inbetween the Corsair-V and the RX-Q). Hopefully one of these will fit you.
When you put a helmet on you need to push from all 4 sides hard enough to where you can feel contact with your head (one side at a time, of course). For the front you'll have to push on either side of the shield. Be sure not to rotate the helmet at all, you want a straight push.
Then push down on the top the same way. You're looking for contact points which will tell you how well the shell shape matches your head shape. Getting contact isn't necessarily bad. What you want to avoid is small contact areas. And like you said, the "crush the styrofoam with a spoon" approach can do wonders. But you don't want to crush a large area or more than about 1/8".
In other words, you want to move the helmet about the same amount to get contact side to side as you do front to back. Top down you're looking for small contact points around the parimeter of your head. Hard to explain but a good fitter (saleman) will get it right.
I can guarantee you there are no helmets made for a triangular head! But I can tell you that a properly fitted helmet is a sweet thing and you can wear it all day long and never really give it a 2nd thought.
If you find an Arai helmet shape that fits you well you can cross reference this to other, similar head shaped helmets. It won't be a perfect helmet to helmet fit but will get you into the ballpark for your search if you don't want to get an Arai.
Good luck, and if I can be of any help let me know.