It has been awhile, so before listening to my .02, consult an expert.
The material used in the draw is generally limited to the width of your draft. For example, you have 6 inches of draw, your draft is 5 degrees, the width of material used in making the vertical wall is a little over a half inch. In other words, you need that half inch to stretch to 6 inches (insert joke here), thinning out the overall material thickness. What will drive everything is the thickness of your foam and what is your minimum wall thickness in the vertical wall. This assumes fixing the position of your material's outside edges. But if you don't, I thought you could get material folding in on itself.
I'm a bit more sketchy on this, but I thought you could use a heat assist with compression molding, alleviating the thinning of the material.
Big radii also help.
But like I said, I basing this on a project a did nearly 2 decades ago. Memories fade. I could be entirely full of shit.