Place the pumpkin in the plastic pail.
Mix the latex mold solution, following the manufacturer's directions, and pour the solution over the pumpkin, covering it completely. Let the mold solution set until firm, about 30 minutes.
Pull the mold from the bucket. Separate the mold into two even halves by cutting through it with the knife.
Mix the concrete with water in the wheelbarrow until the concrete is the consistency of oatmeal. Fill the mold recesses with the concrete.
Put the mold halves together. Tie the molds together with a length of string and re-insert the entire mold in the plastic pail. Let the concrete set for 24 hours.
Remove the mold from the pail. Pull the halves apart and remove the concrete pumpkin. File seam lines with a metal rasp.