Step 1: Supplies
1 2' x 8' sheet of 1.5" thick and
1 2' x 8' sheet of 1" thick extruded polystyrene insulation board. It can be pink or blue, whichever your local big box home improvement store sells.
2" thick foam square (only needs to be a little bigger diameter than your neck)
Metal ruler / tape measure
Face mask or respirator
Extendible Exacto knife (must have enough blade in it to cut all the way through the styrene
Various colors of craft paint
Paint brushes of various size (small craft for the details, larger for the base coats)
Sand paper (100-220, plus drywall mesh)
Optional: band saw or hot wire, circle cutting jig, oscillating spindle sander.
Photoshop or other free layer based image editor
Step 2: Planning the head
After this was all done I drew up the dimensions in illustrator so that I had shop plans to work from and didn't have to keep thinking about what needed to be cut where. These are included in the PDF and will work if your kids head isn't too big.
Step 3: Cutting out the parts
In order to get consistent circles and speed up the process I made a simple jig out of 1/2" mdf and a finishing nail. With the jig in place it was easy to set the distance from the blade and drop squares of EPS onto the nail and spin perfect circles. For the top and bottom caps I also ran the discs through again at a 45 degree to remove as much material before I had to start sanding. I'm sure you could also cut the circles using one of the many 'ibles on hotwires as well, but I had a bandsaw so that seemed easiest.
For the inside circles I used the extendible exacto knife (i'm sure there was a better way to do this, but the knife was pretty fast and the inside didin't have to be perfect.
Step 4: Sanding and Glue Up
After all the circles are cut I used the drywall mesh to rough the rounded top and bottom caps. It works really fast to be careful you don't take off too much material. Then I used 100 and 220 grit sand paper to remove the marks the mesh left and also sand out any bandsaw marks.
After that start applying glue to the top of each ring and then adding a ring. When it is done put 5-6 lbs of weight on the top to hold it in place overnight. DONT glue the foam ring into the bottom. It will be held in by friction. The foam ring is a collar that allows you to get a lego over your head and still have a narrow neck opening.
For the body I kinda eyeballed the openings and then kept refining them until I could get my son into the body. If I did this again I would have done the whole body out of high density foam instead of EPS it would have been easier to use as a shirt.
Step 5: Paint!
I'm pretty sure I covered the whole head with a thin layer of white glue after it had all been sanded smooth. If you plan on using a spray sealer you will have to do this or the propellent in the spray will melt the styrene.
I used craft paint, starting with a tan and adding some pink and brown to get the flesh color. I made a lot since I was sure I couldn't match it, and I didn't know if it would cover in one coat. That being said I used a larger brush and put the paint on pretty thick. It pretty much covered in one coat.
For the bottom foam ring I coated it with white glue first and let it drie overnight. I was pretty low density so it soaked up a ton of glue and paint before it started to look flesh color. It also took a long time to dry.
The brown paint of the body was not as good quality paint and required more than one coat.
Also you can see in the later photo I decided the store bought hat was lame and made a hat out of leftover styrene. I basically did this the day before halloween so It didn't get two coats of brown.
I also ended up making the mouth bigger so my son could see out the mouth (sorta). The inside of the mouth was painted black so the pink foam didn't show from the outside. The last thing I did was coat the whole inside with white glue and then spray the outside with clear gloss sealer. (make sure you keep the can at least 12" from the foam or you will melt hours of hard work.)
On a final note. Although this costume is a crowd pleaser, for a 7 year old to trick or treat in, it kinda sucks. Visibility is bad and it is hot. My son carried his head most of the night and finally took the body off because it was too hot and rigid.
Dense foam would have worked better, but the solution for the head would be hard. On the other hand if you just want to build a giant minifig for a decoration then the legs would be really easy to complete the statue.
Good luck and let me know how you improve upon the design!