Use G1 Octane, because he's a G1 toy and those fit style wise into every line ever since they are so amazingly designed and detailed. Or wait for an Animated Octane, since that style totally doesn't clash with anything else and could even be passed off as a Movie toy.
OK, joking aside, that Universe Tankor really is your best shot at the moment, maybe a 3rd party will make a better one down the line, one that better fits with the new triple changers.
Now, here's something I would love to point out and laugh at.
Remember when Universe Tankor came out and people complained about him being a Deluxe? Remember Hasbro's reply to that? "He's supposed to fit in with Classics, and since Astrotrain is a deluxe in that line all Classic style tripple-changers will also be deluxes to not look out of place with Astrotrain."