This design gets as close to the movie proportions as possible. The arms are fake and slits will be cut on the sides to allow the real arms to be stuck out if needed. Thankfully my wife was happy to be "slightly inconvenienced for the sake of accuracy".
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Step 1: Body Structure
Once the dome has dried on the yoga ball, deflate SLOWLY and remove. Deflating too fast can cause vacuum and it will crumple the head. I recommend you get light weight wall spackle and try to get the dome as smooth as possible with multiple layers of fill/sand/fill/sand. It makes a HUGE difference in the final finish quality. The fleece can't hide everything.
Once the dome is off, turn it upside down and spray lines of expanding foam inside to make a "subframe" to give rigidity. Make sure there is a band around the outer rim. Once cured, duct tape to the top of the body tube. Then climb inside and add a healthy amount of gorilla glue around the edge. This glue expands and soaks into both types of foam. It will get you the strongest bond possible between the two sections.
Step 2: Shaping the Face
I also hot glued metal strapping to the purple minion's mouth to give it even more shape.
These small details will make the costume stand out a LOT more so take the time to give it some personality.
Step 3: Goggles
The eye was made by making a hoop from plastic tubing. Then spandex type material was stretched extremely thin over the frame and tacked in place with hot glue. When installed the wearer will be able to see out since it is dark in the costume, but those outside will see light reflect off the white hiding the person inside. To get the pupils, I found a tube and did short bursts of spray paint to create each circle. Take it slow to keep the paint thin and not impede the ability to see though the spandex.
Eyelids are made with EVA foam wrapped in the applicable body fabric (fleece) and hot glued to the hoop. The whole eye assembly is then hot glued into the goggle at the end. Be careful to not have trapped dirt and clean the acrylic with windex before install.
The straps around the side are strips of 1/2" EVA foam with lots of the furniture nails to look like rivits again.
Step 4: Upholstry
The key to the minion costume is getting the smoothest dome head possible with the least visible seams.
BEST ADVICE: If you're building this for someone else, try to talk them into a theme that hides as much of the dome as possible to make your life easier. If you have the flower lei it will hid seams. If it is the golfer you can avoid the dome entirely.
I used fleece fabric to give some hiding of imperfections while having some stretch to keep wrinkles out. Start by spraying a vertical strip of glue on the front of the dome. Press the fabric and let it set for a couple minutes as this will form your anchor.
KEY ITEM: Stretch the fabric the least amount possible to get out wrinkles as you go. It will take more and more pull to get them out as you work to the back.
Once the dome is done. simply wrap the rest of the fabric around the sides, again starting from the front. If you have a seam or wrinkle...you want it hidden on the side or back.
Cut all holes (eyes, mouth) before upholstering. This way you can then wrap the fabric around the edge.
Overalls: I suck at sewing so I did all my work with a rotary cutter, spray adhesive, and then wrapped each edge over on itself.
Step 5: Purple Minion
I still need to add pipe cleaners to give the "stubble" you see on the little monster in the movie.
Step 6: Bee Do Lights
I added a LED headlamp from Costco that had a flash setting.