Our son, Daniel, is in the middle of this year’s costume. We always try to incorporate his wheelchair into his costume. This year we decided on Audrey 2 from the Little Shop of Horrors. We created this costume from scratch utilizing hula hoops, foam-board, cardboard, PVC pipe, duct tape, spray paint and many leaves and vines from the craft store. As always, he had a blast and really loved his costume.
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Wreck-It Ralph Family Costume with Sugar Racer Wheelchair Costume 
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Awesome Wheelchair Costume: Queen of Hearts and her Golden Throne 
Awesome Hot Air Balloon for Child’s Wheelchair Costume
How did you get that shap with cardboard? It’s perfectly round.