Google "Ruggles scales." Some complain they are not repeatable on weights but I think it depends on how tedious a person is. They can be found for sale occasionally but super simple to build.
I found an old crappy picture from early last year before the front end got ripped off.
This methods could be improved I'm sure but seems to work within 1%. Just make sure the scales are very level on the floor. Side to side is probably more important than end to end.