How to make them:
First of all I bought the 30x40 inch foam board and gridded out the 1" square on the whole board. This make it faster for cutting and painting. You can make them 2" if you want but I loved how the 1" looked finished. Measure the width of the shoulders of the person you are making it for and add 1" to the size to make it an even number. So if the shoulders are 15" make it 16". It helps to make it an even number over an odd number. Measure the depth: meaning the chest to the back. Again a even number is better. Take in consideration the candy bag and ventilation. Measure the height of the person from the shoulder to the floor. Ok once you have all these measurements this is how you would go about making the shapes. I'm using my daughters creeper costume for the measurements to give you and idea of how to make yours.
Supplies: foam board, duct tape (white), strong duct tape (for the inside), paint (white, black and main color you want), magnets, mesh or window screen. 1/2" paint brush, xacto knife, ruler and pencil.
Paint: I bought the snack cups with lids to preserve the paint. One is filled with the main color. One is a light version of the color, one is a dark version of the color. Then I did a light grey and a dark grey. I worked with one color and did about 3 squares of each color. The white of the foam was the last color. For the toes of the creeper I used black and black with a drop of white so it was very dark grey or light black.
For extra effects I bought a battery operated led lights to be placed in a light box and used for the creepers eyes as the kids look thru the mouth openings.
Head: My daughters shoulders was 14" wide. So her head was also 14" and I made a 14" box out of foam board and white duct tape as the the board is white and use the 1" gridlines for you to line up your tape for cleaner edges. You will need 6 pieces for the head: The front head piece you will cut out the eyes and mouth of the creeper. The bottom piece will be an opening big enough for the head to get in but not too big. I made the opening about 4" from the edges. I also reinforced the inside seems with stronger duct tape. Paint the inside black and use black cloth or screen material to cover the openings of the face on the inside. This is your head.
Light Box: So I made a light box using the foam board for my led lights that I bought. The box was 8" x 3". You only need 3 8" x 3" pieces and 2 3" x 3". Make a box with the tape only on the outside on the box. You should have an open side. Tape the battery box on top of the light box and make a small opening for your leds to be threaded thru. Tape the lights up on the top part of the box. Then place the light box inside of your head piece. (This might be easier to do before you put the bottom piece on but its possible afterwards). Use duct tape to tape the box so that the bottom of the box lines up with the bottom of the eyes. The box should be bigger then the eyes so that the lights are hidden and all you have left are a glowing eyes.
Body: The body was 14" wide and I made it 10" deep. I then made it 26" high. How I came to that was to take her height from the shoulder to the floor. Minus 8" for the height of the feet and another 4" for the clearance to the ground. You need 4 sides. 2 that are 14x26 and 2 that are 10x26. You will need one more piece that will be the top cap to the box where your head will stick out. Measure the inside of your box and tape it together. Cut and opening in the top for the head. Make it 2" to 4" from the sides. Put velcro squares on each of these corners. This is your anchor point for the head. Match your head up the sides. It should hang over about 3" or squares. Add the velcro to the top of the head. Now you can take the head off and on. After your body is made now it time for the doors. This is why its nice to have an even body. Measure down about 5 squares from the top of the body and the bottom of the body. The doors will be the remaining squares. Cut along the the width of the body, all 14" and then the down the middle. The size of the door is up to you. Now you should have two swinging doors. At this time reinforce the inside seams with stronger tape. Just don't put any on the door. Paint the inside black. For the door handles I just used the stronger duct tape, folded one piece in half and used two others to tape it down. To make the door close, I bought 1" long magnets I found at Lowe's and Home Depot. I super glued them on one side until dried and then matched them to the other side. This is on the inside of the door. Now when the doors shut they stay closed (hopefully). Paint the squares and the body is done.The feet get attached after they are done painted.
Feet: The feet are the most complicated part. There might be a better way but this is how I came up with mine. Ok so they will be 8" high or 10" high for a taller child. The width of the feet should be the half of the width of the body. So for my daughter, her feet were 8"x 7" for the front piece and 8"x8" for the sides. This is cause the creepers are squares but people are not. Ok so take your 3 sides and tape together. Make a top piece that is 7" width and 5" deep or 3" smaller than your full size. Tape that to the top of your foot. There is another piece that goes on the side of the inside of the foot. It is 2" wide and 5" high. This is taped to the back of the foot with the tops matching and the gap at the bottom. Ok now you make another piece that is 7" wide and 10 to 12" long. This is the piece that your childs leg will hit against to make the feet move up and down. Tape this piece to the bottom of the foot and angle it up to hit the 2" back guide. So you basically have this piece sticking out the back of the foot at an angle. Paint your feet but leave the last two rows on the front of each foot for the toes. They will be painted black and dark grey. Place your body down on a table. Your feet should be place on top of the box and alined with bottom of the body. Tape the inside with the strong tape. Your foot should be able to flop down and stay in place.
Painting: Paint the squares on the creepers with about 3 shades of the chosen color (the color, and then one with a little white and one with a little black) and 2 shades of grey, one light and one darker but not black. Just alternate your squares. There is no set pattern.
Now your creeper is done. To put it on your child, lift the feet up and place it over the child's head and then let it slip down till their head is thru the top. Then add the head. You are done and they can walk and surprise everyone when their doors open up.
You can add sound effects or lights. Also the child is free to where anything thing they want in the inside of the costume or even wear another costume. As you can not sit down in this costume.
Hopefully you can follow what I have written down. It took a lot of work and tweeks to get it to where it is.