Paper mache made with PVA glue and tissue can be extremely strong, even in thin layers. When you have removed your stuffing, test your pumpkin by pressing it gently. It should hold its shape. The ridges you made with the thread will help to make it stronger. If you need the lid to be thicker or the top of the walls to have an edge to hold the lid, you can add more tissue on to the shape, see the next step.
If your little pumpkin barely holds its shape, apply a coating of glue to the inside of the pumpkin and begin to add more layers of tissue. Be careful you don't add to much glue or the structure will become too damp and lose its shape.