To make the transition form, I started by gluing a ridgid foam block in each end of the two halves of the snorkel and then sanded them roughly to size. I then aligned the halves and started to sand in between the blocks. I continued until the block faces where parallel and the dimension between them was correct. I then glued the blocks together. Note that the blocks are fitted into the ends of the snorkel pieces by about 1/4".
