Interesting that we are just now installing a new belly pan on a 1959
Tradewind. We are doing a complete shell off restoration and have the facilities to lift and rotate the frame so I have the luxury of working with the frame upside down which makes everything a lot easier. We have found that it is really not too difficult to get the belly pan at the frame ends to come out looking good.
I gather that you do not have a belly pan to use as a pattern?
Here are four pictures of the detail on the rear and the front of the bottom of the trailer at the frame ends. I also took a picture of the old belly pan laid flat on the floor showing the rear detail. Notice how the V is cut. It is not quite as you would picture it so if you have some scrap metal (preferably thin and easy to bend) I would recommend that you practice first.
The last picture is of the frame with insulation but no sheet metal. It is taken from the front of the trailer (again the frame is upside down). You can see how we heavily modified the frame and added a spare tire carrier (and stainless holding tanks). The spare tire area will not have sheet metal over it, so it was easier to make the front belly pan out of multiple separate sheets that we will rivet together instead of one sheet. This is why the front detail does not look the same as the rear.
One other detail that is different on this trailer. I was able to get the belly pan aluminum in 5 X 10 foot sheets but only in .045. It is 5052 H32 but thicker (normal belly pan aluminum is .025 or .032). It is much more difficult to work with but since the frame is upside down, not too bad. The advantage is it lays flat very nicely and should wear very well.
These pictures are probably confusing, but hopefully you get the idea.