I don't know what level your "construction expertise" is at, but the simplest way I can think to make a representation of a basic cash register is to cut two identical sides from 3/4 inch plywood and then assemble the two sides with corregated cardboard (the width of the cash register) by attaching the cardboard along the edges of the side pieces.  If you use foamcore, you will have to do it in sections since it doesn't fold cleanly without scoring (and that takes a very sharp knife, a straight edge, and a little skill).  Make sure your starting and stopping location is the bottom of the "box" (cash register), preferably the bottom, rear seam of the cash register.  Finish up by taping (1 1/2 inch or 2 inch masking tape) all the seams and joints and then painting it.  You could give dimension to the "keys" by gluing on buttons, but merely painting them on would probably work.