Plywood training plates
I made these training plates a few years ago as a way of getting the Olympic bar to the proper height for deadlifts, cleans etc. but without putting any weight on the bar - especially when training new clients.
Simply get a piece of plywood, draw around the outside a full size Olympic plate and the centre hole, use a jigsaw to cut it out and you have a very cheap but very effective set of training plates.
"50kg" training plates