It's really not a "bit much" to call it an insult to all commissioners when you're essentially saying "I put no value in your work, but do it for me anyway."
Design is WORK. I know a lot of people are under this bizarre impression that design work "doesn't count" or whatever, but an artist's time has value, the same way a mechanic's time does, or a plumber's time does.
Would you ask an architect to design a building for you for nothing? They're not building the building, after all, they just have to figure out down to details what it would look like according to the client's desires, and make sure that it would be possible to achieve in reality without bending the laws of physics or collapsing in upon itself or being completely out of the client's budget.
That's the sort of work you're asking for here, just replace the "building" with a helmet that you're presumably going to be wearing on your head, so I assume you want it to be functionally possible. An artist's work might not be the actual building stage, but it's still worth money. It's a job, that takes time and thought and knowledge of their craft. It's not a doodle.