Making the Head
The head is made from a 10" sonotube (the kind used to cement in fence posts), about 13-14" tall. Although if you have a big melon you might need to go up to a 12" size. I "tried on" the various sizes in Home Depot when no one was looking to get the best size for me. The sonotube also supports the entire costume by resting on your shoulders on foam pads, so make sure you leave it long enough to trim to to its eventual fit.
To make the dome part of Bender's head, I inflated a balloon and stuck it out the top. Then I applied five or six layers of paper mache to make the general shape. Paper mache shrinks and leaves a rough finish so I used drywall mud to get the dome smooth. It's extremely difficult to use regular flat drywall knives to get the rounded contour, so I cut out a piece of cereal box that I could hold in my hand in a curved shape, and that did a great job smoothing the mud. I hate drywalling around the house, but I got a pretty good result here with 4-5 coats and a bit of sanding.
After the dome was done, I figured out the location for the eyes and mouth. I basically left a 1" gap from the top sonotube cut line to the eyes, then another inch between the eyes and mouth, and another inch between the mouth and shoulders. The eyes and mouth are close to the same shape and dimensions.
To cut the mouth out of the sonotube I used a jigsaw with a narrow, sharp, blade and went slow. Sonotubes are made of dense thick cardboard, and while it's possible to cut them with a sharp razor knife, you have to press really hard and make multiple passes, and if you've ever done anything like before that with a knife, you know that it's a) dangerous, b) it's impossible to make smooth curves, and c) the knife likes to run away and go too far. The jigsaw gave complete control and made a nice smooth cut, easy as pie.
To build the eye frame, you need to start with an oval base and then wrap it with cardboard. The base will also be what you glue the eyeballs to. I found that two mixed nuts containers side-by-side provided the perfect size for the oval base. I cut the oval out of ΒΌ" plywood, although cardboard box material would work just as well.
Then it's just a lot of finicky work trimming it to get the right contour to match the sonotube's curve. You can put the frame on the edge of the sonotube and trace it to get a starting point.
When it was done I 'sealed' the outside edge of the eye frame with my glue gun (below-right pic), just so that it wouldn't look so obviously like the edge of a cardboard box once painted. Then the inside gets painted black.
The eyes are 3-1/2" Styrofoam balls cut in half. The pupils are just square pieces of black electrical tape. The mouth is made from white fabric mesh hot glued to the inside of the sonotube as seen in the below-right pic. I found this mesh at the dollar store as part of a toiletry bag. The teeth are drawn on with a black sharpie. I had no problem seeing through it.
The cigar is made from a 3/4" wood dowel with a neodymium magnet hot glued to the end of it. It sticks to a piece of metal coat hanger wire hot glued inside the head. The squiggly part of the wire holder in the pics below is just to provide more surface area for the glue, and to also prevent it from twisting.
The antenna is a 3/4" dowel sanded into a cone shape with a ping pong ball hot glued onto the top. I chose to use the magnet mounting method again (with a 5/16" flat washer glued to its base to stick to the magnet), because this is a tall costume and I was worried that if I glued the antenna on solidly, I would break it off on ceilings and going through doors. And sure enough, I knocked the magnet antenna off many times.
