Coffin: The design is for a 5ft 11in person and is made from 12mm untreated ply, 120cm by 240cm. 9mm ply could be used with a little extra reinforcing. Two coffins can be made from three sheets of ply. Check your local crematoria rules. In Timaru, nails and screws may be used. It would be easy to remove screws and replace them with dowels and PVA.
Bumble-bee box: Bumble-bees are excellent pollinators. They do not sting and can be encouraged by placing assorted boxes in various locations. The queen bee will hibernate underground every winter. The boxes should then be sterilized or burnt.