John,
I used 3x3 1/2"x 1/4" steel plates. Also with the exception of the very front 2 I made sure that one lip of the backer plates overlapped the c-channel underneath so that it had additional strength. I didn't do the front ones that way because they didn't use c-channel up front. I may end up changing them to drill down through the aluminum tube but I don't intend to use them for the tractor so I am not sure.
Here is a picture of the backing plates I used. If you would like I can snap a picture of them under the trailer. I also used some foam between the steel and aluminum to try and prevent corrosion.
I used 3x3 1/2"x 1/4" steel plates. Also with the exception of the very front 2 I made sure that one lip of the backer plates overlapped the c-channel underneath so that it had additional strength. I didn't do the front ones that way because they didn't use c-channel up front. I may end up changing them to drill down through the aluminum tube but I don't intend to use them for the tractor so I am not sure.
Here is a picture of the backing plates I used. If you would like I can snap a picture of them under the trailer. I also used some foam between the steel and aluminum to try and prevent corrosion.