After finding the excellent work by Raynquist here: https://www.reddit.com/r/factorio/comments/a3awpa/finally_a_smaller_1616_balancer/ I decided I wanted a red belt version of his 16x15 inline balancer.
Most of it translated just fine, but there were two undergroundies that stretched just a little too far for red belts.
One was a trivial fix, but the other gave me some trouble - I eventually had to extend the length of the whole balancer by one tile so that it's now square at 16x16 tiles.
This is the result of that effort.