I don't know how big your 'car trailer' is, or how big you want your box, but a very strong and secure box can be purchased at HD or Lowes, other similar stores, and it is called a 'job box', a tool box used by contractors so what they can store tools at a job site and not carry them in their truck. You can bolt it to your trailer floor and it has 2 places on the lid for recessed padlocks, constructed such that you cannot get to the hasps with a bolt cutter, so it is secure. Reasonably priced, various sizes (small, med, large, huge). These boxes are MUCH tougher than any alum brite tread you'll find for sale.
I made one for a trailer I used to have and put in my generator, batteries, and lots of room left over for golf clubs, chairs, other camping stuff. I cut an air vent into the back (so the generator could run with the lid closed if raining) and had an exhaust pipe (made of schedule 80 plastic pipe - it won't melt, really!, not even after hours of running your genny)to vent the exhaust.
