If your current trailer hitches up near the drop out instead of the seat post, then possibly try some Donkey Boxx panniers. They are tested to 30# capacity, and the lids provide a wider, flatter area than the rack platform. And you could also wedge something into the space between the boxes. Use cargo nets as you see fit.

Edit: That weight rating is non-scientific, performed by the designer, and for one unit. Double check the weight limit on your rack, as some of the cheaper ones aren't rated up to 60 lbs.