This is the last year we are doing a Jack of Hearts Costume for our son Jack. All I did this year was buy 2 white poster boards. Then I drew the front of a bicycle card on one poster board and a Jack of hearts on the other poster board. It was not too difficult to draw it free hand (and I am not an artist).
Coloring it in was time consuming though and I would have spare markers on hand (because mine did dry up). Then I poked holes in the tops of the poster board and put some string through it so when my son wore the poster board it would be worn like a sandwich board. Make sure to measure the poster board before drawing because if it is too tall they will have difficulty walking.
I just let my son wear his regular clothing underneath (which suits all sorts of weather conditions).