Start with a skull!
If you're a reasonable person, you'll stop there.
If not, why not make a whole skeleton out of pumpkins?
Finding the right pumpkins can be quite a challenge. You'll have better luck with a rural grower with a variety of shapes and sizes... the soulless clones that make it to your supermarket are just too perfect.
You'll need five separate pumpkins:
1. The skull: Shaped like a pear, and as big as your head
2. Torso / ribcage: Tall, barrel-shaped, or large pear, and torso-sized
3. Pelvis: spherical (a soulless clone will work for this one).
4. Limbs: As tall as your femur. The taller gourds tend to have thick skin and are stronger than pumpkins with poorer posture.
5. Feet & Hands: spherical (clone OK). You may be able to use parts from the other pumpkins for these...
It's an odd experience to hunt for pumpkins with this twisted agenda; you're not looking for perfection, you're trying to see the potential for body parts. But the nice thing is, these are usually pumpkins that nobody wants, because normal people don't make skeletons out of them.
Of course, you're looking at some serious total weight by the time you find all of your pumpkins, so be prepared to pay for well over fifty pounds.