I tried several times to make a plaster caste out of a pumpkin and the first few times ended in disaster. I mean real disaster... plaster gushing off the table and onto the floor! I wasn't able to make a liquid-tight wood frame in the size I needed for a pumpkin. I decided to throw away all I was taught about how to make a plaster mould and try something innovative, I cast the entire thing in a plastic bucket. Here's what I did.
Firstly, I soaped up my pumpkin so that the plaster wouldn't stick to it. I also did the same thing to the bucket, but I don't know if that was necessary because the bucket is smooth plastic. I needed to find a way to hold up the pumpkin at the bottom of the bucket so that plaster could flow underneath the pumpkin. What to do, what to do... I devised an interesting solution of using the plastic pizza "tables" that one gets with a take-away pizza. The pumpkin couldn't be too heavy for this to work and it required me piercing the tips of the pizza table into the pumpkin.
I then put the pumpkin into the bucket and filled it with plaster half way up the pumpkin. When it was semi-dry I inserted tiny "pill" shaped clay into the plaster. These would be the guides into which the top portion of the mould will fit once it was done. I also added a clay cylinder at the top that would stick out of the top part of the plaster mould and this is where I will pour the casting slip in the future. Once the bottom layer of plaster was dry I removed the clay "pills" from around the sides and I soaped everything up again. I then poured the top half of the mould and was careful to keep the top part of the clay cylinder sticking out.
This is what the final mould looks like. I use strong inner tubes from a bicycle tyre to hold it together.
And this is what I am using the pumpkin mould for, I am making pumpkin candle holders. I add the twisted vine by hand after the pumpkin is cast. I also create an open space that is shallow and wide enough for an extra large tea light.