Some thoughts. Choose your material. Solid timber looks nice, there may be a shed member who can help joining planks to make the required width. MDF is cheap but heavy and is usually painted afterwards. I chose 12mm untreated ply for its lightness and strength.
Try bending an offcut. This plan is based on my trial. The head end is a little wider so that extreme bending is not required.
Three sheets of plywood will make two coffins. The lids are wider than the base, so draw one left to right and the other right to left.
Storage. Mine will be kept vertical in the shed with shelves slotted into it. As it’s untreated, the back will have a solution of borax and anti-freeze washed on.
A person’s height is normally measured standing up without shoes and to the top of their skull. Lying down with shoes and room for their hair requires another 2 to 4 inches, say 50 to 75mm.
This plan is based on a sheet of untreated 12mm 5ply 120×240 and is for a 180cm person. A 6ft (183cm) person might just fit.
Cut the two long sides off, 30cm wide. Mark and cut the other pieces. Make appropriate adjustments to suit a person other than 181cm. TIP: if possible, have the customer lie down on the marked base before cutting. It is recommended to add at least 5cm to a person’s height when designing the base.
Kerf along the back at the dotted lines. Two parallel cuts leaving only a one ply thickness are required. Practice on an offcut is recommended. Also practise steaming and bending.
To be continued…
Another shed member is also making a coffin. He has bent the side without steaming by using multiple kerfs.