The standard hitch is only rated for 120 LB unsupported. If you are pulling a trailer w wheels on the ground it can support 500, but only 120 for a box w no support. You are going to break something.
From the manual...
Carrying Accessories
The Model X towing package has a 2" hitch receiver that can support an accessory carrier.
The hitch receiver is designed to support vertical loads of up to 120 lbs (54 kg). When carrying bicycles or other items on the Model X hitch, always check to ensure that the maximum weight is not exceeded. When calculating weight, remember to include the weight of the accessory carrier. For example, assuming the carrier weighs 40 lbs, the weight threshold is sufficient for carrying two bicycles weighing approximately 40 lbs each, or four bicycles weighing approximately 20 lbs each.