My easy way of making sand casting flasks with cheap tongue and groove boards. And casting some custom knobs for an upcoming project. The advantage to use tongue and groove boards is that when locking togheter they help tremendously to line up the two part of the mold, I made all my recent flask this way and never had misalignment. The tabs on the side are there to lock the two mold parts togheter to prevent mold separation caused by molten metal pressure (probably not needed for a small flask like this one but better safe than sorry). Spray pait of contrasting colors works better than lines or numbers on the sides for me to prevent putting back the mold togheter in the wrong way. Wooden strips nailed on the inside helps to keep the sand inside the mold for flask that carry some sand weight. In this case was needed only for the thicker part in my opinion. NOTE: I forgot to show you one step, I added a couple screws on each corner of the flask to add rigidity to the frames. Screws lock way more than nails!