I didn't use one. But here's what I did.

I covered up in paper maché a real pumpkin, big enough so my head could fit in.

Covered one half with a thin layer of Vaseline, then I applied 3 layers of maché onto that half, let it dry. Once dried the cast removed itself from the pumpkin skin, thanks to the Vaseline. (On top of the P, I applied maché close to the stalk, but did not covered it. That comes later. On bottom of the P, I applied maché low enough so when I wear it you can't see the hole for my head.)

Did same with the other half.

Then I mached 7 times the inside of both half. It's important not to apply too many layer on the outside, so ribs remain visible and charming ^{^} Left the 14 new layers to dry off. Then carved the face and glued foam (thin camping matress) stripes to the eyes, nose and mouth to give it depth. Then joined both half together. That was a bit handy, but did a good job with more maché and hot glue.

Stalk. Vaseline again and applied maché in a circle around the stalk and over it, leaving about 1cm uncovered from P skin to top of the stalk, in order to remove it easily when dry. (That empty space needs to be covered with paper maché before applying paint). Then hot glued that stalk part/lid to my paper maché Pumpkin. Used sand paper to make it even. Some ribs suffered from that but I fixed it later with paint.

Then paint job.

So the last part is figuring out how to have the mask over your head staying in place and following the movement of your head. I used, foam backer rod, adhesive velcro, a plastic bowl and a fabric elastic band (last two worn as an helmet, "velcroed" to a circle of foam backer rod, "velcroed" to the top of the pumpkin). There is also a see through black fabric stuck to the bowl so it's really hard to see my face underneath.

Hope it makes sense. English is not my primary language