Looking good so far. I would do like Tommy said and just make a seat. Trying to make an existing one fit would be a pain plus you could make it look however you wanted (Color, stiching etc.). I would cover the side rails and have it go up to the gas tank. Instead of doing a two up seat I would do a real low profile seat that just extends a bit onto the fender. Kind of like the ones below. It's still a two person seat but doesn't have the big bump like a regular two up seat. May flow a little better. Just an idea.

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