We wanted to do a really original costume and found one of a Lego man head on the internet. But there were no instructions so went ahead and made our own Homemade Lego Men Group Halloween Costume.
We bought several sheets of Styrofoam and cut out circles with smaller circles cut inside them and stacked them up with glue and broken coat hangers put through them for support. We then sanded the outside with a fine sand paper and a file to get the smooth rounded surface. We used some of the excess foam to make the top notch on the head.
We brought a Lego man down to a local d.i.y store and matched the yellow and painted on the head. The inside is lined with duct tape to stop you breathing in bits of foam. The major set back is you cant really see with it on.
The bodies are just card board boxes painted with the same paint and muscles drawn on.