My son loves calculators and when Halloween was coming up a few years ago we told him he could be anything he wanted to be. Of course he wanted to be a calculator. I knew it would take a little work but it wasn’t so difficult.
We used a box and cut it down to size and fit. Then we painted the box gray and finished the edges with black duct tape. I picked up reflective numbers at Home Depot and used a silver sharpie on black construction paper for the display screen and function keys.
Everyone knew right away what he was and he was so very proud of his calculator costume. The only trouble he had was being able to put candy in his candy bag with one hand while holding the bag with other. He loved this costume so much we had to talk him out of being a calculator the following year.