Hi Alan
the uprights for the front beam support and rear are 40x40x2. 4mm is way to think and heavy. On my first trailer which i am still using and 40x40x1.5. Its been going for 6 years now and travel everywhere, but i think thing 1.5 is a bit underdone. We are happy with 2mm wall thickness. The base is 3mm wall thinkness,and we uused RHS not SHS, as square sections bend over time over the axel

Matt