I've never tired using a cleaned out pumpkin for a mold but I don't see why it wouldn't work. I can see that in some instances that the pumpkin would stick. I suppose you could cut it off or maybe use a plastic dry cleaner bag for a liner. I'm not a big fan of the plastic bag liners because of the lines they leave.
You could also take a shallow bowl and once the cement mix has set up, unmold and carve out some grooves in the top.
Yep, you can use those caution cones as long as you spray with a mold release. I have a tall vase with a similar shape that I use for stems. Billie

Thanks for the reply billie...this is on my "to do list". I thougth the pumpkin would release much like the leaves would in leaf casting but maybe you are right and it would stick somewhat. If I can I would like to give this a try this year...all the pumpkins are on sale right now haha. Quickly running out of time to get things done though, might have to wait till next year.
Thanks again.

Not all leaves release easy; some stick so bad you have to let them rot away. I tired blasting the remmants of a real leaf from a casting before it was cured. I blew a hole right through the casting. So even if the pumpkin sticks and you don't mind rotting pumpkin, go for it.
And don't throw out your broken leaves, you can add them to your other garden planters or scupltures. Billie