It is flat bar, I cut it from a sheet on my tablesaw. I TIG welded the joints, just working out the bow and stern now. It was what I had in stock, and I thought it might be a fun way to do it.
I don't really want to use mechanical fasteners for several reasons if I can avoid it. I don't want to weaken the stringers, I definitely don't want to use a fastener of another metal because of electrolysis, and I like the elegance of lashing. That said there is no "stop" on the frame and I am a little concerned that the stringers might want to slide on the frames. I could weld on a small aluminum stop, but I am hoping to avoid the extra hassle. I'll see how it feels when I start lashing.