This project was built on the fly, so I don't have full plans for it, but this will at least hopefully get you started on your own curved-top casket. First, a note about the measurements. 1" X 2" boards in most cases actually measure .75" X 1.5", thus the measurements are assuming the boards are that size. If you get boards that actually measure 1" X 2" boards, simply add .5" to each board in the measurements. The bottom half of this coffin is the easiest to build as it is simply a box. Decide how tall and how wide you want your coffin and then build it out of plywood. I suggest using rafter brackets in the lumber section of Home Depot to fasten the sides together. I also suggest running a bead of Liquid Nails construction adhesive to strengthen the joints. Once you've built the box, use 1" X 2" boards to add details around the outside edges. Once you have completed this portion, measure the width from edge to edge (be sure to include the additional 1" on each side if you added the boards.)
The curved top is acheived by cutting out 5 curved forms from 1" X 6" pine board. The curve is entirely up to you, I suggest getting cardboard and trying various heights and curves to get one you are happy with. The width of the curve form needs to be 1/2" less than the width of the box you just built so that the lid will fit flush once you have added the plywood. Once you have chosen a curve and drawn it on your wood, add a 2" tall rectangle on the flat side. Now you will have 2" of vertical and then your curve. Cut this shape out and then cut a 3/4" X 2" rectangle out of the outside edges of the rectangle you just added. This is where you will add 1" X 2" board to make the side rails of the lid. Click here to see the curve form.
Decide how long you want the upper portion of the coffin to be compared to the bottom portion. This is the panel that will open, I suggest using about a 2/3 ratio making the top portion 1/3 of the total heigth. Cut two pieces of 1" X 2" to this length and glue and screw your 1" X 2" into this notch, making sure it is sticking 1/4" out of the notch. This will allow the curved plyood to sit flush against it. Now you can start bending your plywood over the two curved forms that will make up the top panel. Use wood glue and then screw the plywood down to the curve every few inches to make a smooth curve. The easiest way to determine how big a piece of plywood you need to cover the curveis to screw one side in and then bend it over and clamp it down, then mark where to cut. The bottom panel is built the same way, only use the fifth form you cut in the middle to maintain the curve over the longer distance. Once you have secured the top to the forms, cut off any excess and sand everything smooth. If there is a gap between your curved plywood and the 1" X 2" rails, you may want to fill it with wood putty or the Liquid Nails.
Off the shelf hinges will attach the lid to the bottom and you can purchase a mechanism to keep the lid halfway open (like a ladder hinge that locks open.) Paint your casket to your liking and feel free to add details such as handles made of inexpensive towel racks and metal tape to give it a nice shiny detail at the bottom. For upholstery, purchase a fairly fluffy quilt or bedspread at a thrift store and a large sheet. I simply stapled the bedspread down first (to give it some cushion) and then stapled the sheet in on top. You could certainly go with PolyFil or some kind of stuffing if you want, but that could get a little expensive.
NOTE: 1" X 2" board is not always exactly 1" X 2", you most likely will find your boards to be .75" X 1.5". Be sure to measure the wood you bought before cutting anything.