I used the Casa de Sade plans which allow you to build a toe-pincher style coffin out of one sheet of plywood, definitely good if you are on a budget. I suggest using a circular saw to make all the cuts outlined in the drawing. Keep in mind that one of the long sides of the coffin will be one piece, while the other will require you to put two pieces together. There are a few ways you can construct the coffin using these plans. First you can buy a few 8 foot lengths of 2" X 2" board and cut it into small pieces. Screw these pieces into the bottom of the coffin and the side, to hold the sides up. After doing that, run a bead of Liquid Nails construction adhesive along every seam to strengthen it. Another method for building the coffin is using rafter straps. In the lumber section of Home Depot you should be able to find these and they come in 90 degree angles, cross straps and all kinds of helpful shapes for putting the coffin together. They run about 20 cents a piece, so once again it is a cheap way to go.
The final touch and detail on the coffin would be to purchase about eight 1" X 2" boards (about $1 each) to use as accent pieces. I simply cut these to length and added them along the top and bottom edges of the coffin, a few running vertically and then all the way around the lid. I didn't measure anything, just made sure they were pretty symmetrical, but feel free to review the pictures for placement. The interior of this coffin was done on the cheap as well. At a thrift store, purchase a fairly fluffy quilt or bedspread 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.