Luckily we had some large cardboard boxes for recycling sitting in the garage so I found one I thought would be the best shape and taped it together. I knew were were going to paint it so didn't want to use ordinary sticky tape as the paint wouldn't adhere -instead I used black "electrical" tape, which would blend in with it being painted black. Then I set the kids to work with the paintbrush.
After the paint had dried, I cut out some letters (from foam sheets) and stuck them on the front, along with a counter & some buttons / dials. You could be much more elaborate than I was but I had two little people dancing around me waiting for the end product!
I used an old spray cleaning bottle and a piece of hose to make the pump. I cut a hole in the base of the bottle and slotted the hose into it - it was quite a tight fit so I didn't need to worry about it falling out. Then I cut another hole in the bottom of the cardboard box to slot the other end of the hose into. Finally, I cut a large hole in the side of the box to hook the "pump" into.
I had intended to make the sign on top swivel round with one side reading OPEN and the other CLOSED but I found that it wasn't sturdy enough to withstand the turning so I taped it into place with just the OPEN side showing. This was made from two oval pieces of card stuck together with a straw taped between them...the straw was the weak link, it was too flimsy! A stick (or even a pencil) would have been better. I wrote the OPEN sign on with glitter glue.