I am trying to find some aluminium U channel say 50mmx25mm with a 3mm thick wall. I then need to find some rectangular box section that is a nice sliding fit in it. Height of the box not too critical - it can be a little taller. About the only size I can find is too big and with 8mm thick wall - total overkill. Any ideas? Aluminium seems to be a horrendous mix of metric and imperial.