I don't know if this will work for everyone, but my approach to having a previously owned kit registered with no bill of sale trail at all, was based on a guy's post to another discussion group years ago. He simply checked the box - assembled from miscellaneous parts. I did the same and was prepared to show all that I had contributed hoping it would be sufficiently miscellaneous, but the question was never asked. I guess, I would use the Kitfox Aircraft letter as a first choice if I were to do it again. It did cause a bit of uncertainty. One thing I would definitely do if I were to do it again would be to register it much sooner, maybe six months or so before completion.
Lowell