Aluminium might be preferable, but it's an elegant way to get a body for your trailer, and 7-8 kg is not too bad. Ensure that the lid seal is waterproof. You could always bolt some rails on the top, so you can tie more gear on top. There may be an advantage in having a lid that will open right over 180 degrees, and lock in that position to form a table, which you could use to to boil a cuppa on the side of the road, or keep some of your stuff off the ground.
My two bob's worth.......