I'm always going to try to do Star Wars mashups for these races. I miss those runDisney races so much!
For the Cars 5K I did a Cars / TIE Fighter mashup that I'm calling TIE Fight-car. I wore a Cars-inspired black shirt with a fun Run-eze spoof logo on the back. I used a glue gun to attach the cockpit window to the front of my shirt (a disc made of silver posterboard and glitter foam pieces).
I carried a pair of TIE Fighter wings I made out of silver posterboard, black foam board, silver ribbon, and silver duct tape. I used a pair of old running armbands to carry them. Velcro hooks attached to the phone window on the armbands stuck very nicely to fabric loops on the wings, and made it easy for me to put the costume together myself after I arrived at Epcot. I held onto the bottoms of the wings with my hands to help keep them in position. I was worried about them staying together and figured the wings would get completely ruined if it rained, but we only had a bit of light drizzle before the race started so that was no problem.
I wore a black checkerboard Sparkle Skirt and a red Sparkle Athletic Headsweats visor to complete the mashup outfit.