Here are a few suggestions that I have found to be important in the buidling of a box.

Inside length should be 3-5 inches greater than height of the deceased. Outside width should be no more than 30" if burial is chosen and grave liner is required. Most handmade boxes are a simple rectangle, 80 " tall and 24" wide. Many handmade boxes are overbuilt and oversized.

The bottom of box should be flat for a box that will be used for cremation.

Box is required to be structurally sound and leak proof. Use at least 1/2 inch plywood for the walls and 5/8's plywood for the floor. I would recommend glueing and screwing all joints. Any gaps can be filled with suitable sealant.

Handles need to be strong in order to be safe. I would recommend hardware that is bolted through the box. A simple alternative is rope, pulled through a hole and knoted at the end.

There are numerous websites that provide additional direction and complete plans for hand-made boxes. Ensure that you have enough time to complete the box and the support of key family members before you undergo the project.