I used cardboard and poster board for the costumes. I had to measure each of my children to make it the right size. It was some what complicated when trying on the body of the Lego my son got stuck and could not get out. Then I forgot to cut one side for the arm to come out.
It was fun we had a good time working on them together. When we went to a Halloween party people wanted pictures of them it was like the paparazzi. Just taking pictures out in my front yard people were amazed. Saying AWESOME!! WOW! BEST COSTUME I EVER SAW! VERY CREATIVE. One is a Tomahawk Warrior,Matt Kemp,& Punk Rocker Lego. My boys collect Legos and Lego figurines. They actully have these exact Lego guys.
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Awesome Lego Benny Costume from the Lego Movie 
Awesome Lego Ninjago Minifigure Costumes 
Cool Lego Minifig Costume 
Amazing Lego Men Horror Villians Group Costume! 
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Everything is Awesome, a Lego Movie Group Costume Journey! 
Homemade Batman Lego Couple Costume
This is a great idea….defiantly giving this a try