Well it's been a while since I posted but the frame building is underway
The photo is little misleading, looks way bigger than it is. I went with 8 inch Channel similar to the original only taller. Same gauge, same flange.
Will use the majority of the original A frame, extend the trunk a bit and leave a gap between the back of the trailer and the trunk so the water will pass by
Thanks for all of the advice here particularly from the engineers and airframe guys. Makes experimenting so much more fun. Will keep you updated. Thanks Again