My 5 year old wanted to be a monster truck for Halloween last year; not wanting to buy something we decided to make are own homemade monster truck costume.
I used various sizes of card board boxes; cutting and piecing them until they resembled a truck. He wanted to be a red truck so I spray painted the boxes red. The roll bar across the back was a pool noodle held together by PVC pipe and duct tape then sprayed silver. The hood piece was a shoe box shaped like a spoiler painted black and the knobs were soda bottle caps sprayed silver.
We decided to make the truck roll since he is in a Halloween kids parade at school, I used wooden rods sprayed black for the axles and the tires are Subway sandwich trays sprayed black. I glued Styrofoam inside the trays to give a little stability and used bathroom caulking to glue the top and bottom together so they wouldn’t come apart while rolling.
The hub cap is a blank CD which was held on by the screw and washer that was holding on the “tire” onto the wooden axle. We painted flames on and applied some stickers given to him from a local shop that makes monster trucks. He just wore jeans and a t-shirt that we made with his truck team name on the back. He won 2nd place at the costume contest, he was really proud and loved rolling (it actually did roll) around in the parade