We don't have any surviving sarissa hafts. There are two main choices for pike hafts: one-piece wood, and laminated hafts. Later European pikes, of which we do have surviving examples, have one-piece hafts. This is probably how sarissa hafts were made. There are two methods for making one-piece hafts:

1. Wood as it grows - cut a sapling or coppiced pole of suitable length and diameter. Peel off the bark, and straighten and smooth as required. Straightening is done by heat, either dry or wet (steaming). This produces a tapered haft, which is good for a pike, where you want to minimise the weight in the far end of the haft, while keeping the butt half thick enough for adequate stiffness.

2. Cut a larger (i.e., thicker) tree, and split the wood along the grain. When you have a square piece a little bigger than the desired haft, straighten as required, and shave it round (e.g., with a draw knife).

There are suggestions that the sarissa haft was often made in two pieces, joined by a bronze sleeve. This is based on a find that included a butt piece, a mystery bronze piece identified as such a sleeve, and a head (identified as a sarissa head, but too big and heavy to realistically be one). There are many problems with this sleeve suggestion, and one-piece is more likely. Given that various peoples around the world successfully made and used one-piece hafts, and two-piece hafts with such a joint are quite absent, most pike users decide that one-piece is better.

Laminated hafts have been used. These are made of multiple pieces of wood and/or bamboo, glued together. The examples I know of are East Asian (Chinese and Japanese). The laminated hafts are often wrapped with silk or other thread (or sometimes wire), and then lacquered.