The trick is that the buck is taller than the height of the hood, and that I used double-sided sticky tape and Lego to elevate the buck on the vacuformer platform. Then the heated plastic pulled right down underneath the edge of the buck, nearly surrounding it, and all the webbing was away from the needed form.
I cut the buck free afterward; because the buck is taller than needed, I had the plastic available to do that at the bottom. Then, trimmed to size.