The idea came of extending the bed in the Y direction to 400mm.  It would require a longer track, longer belt, & longer hot plate in the Y direction only.  It would allow the lion kingdom's largest panels to be printed in 1 piece.  

A rectangular expansion requires far less new parts, takes far less space & would be far more used than a square expansion, yet only square expansions are made.  The cheapest 400mm one costs as much as a new printer.  

https://enderextender.com/collections/ender-3/products/ender-extender-400-for-ender-3-non-pro

It could be ground down to 400x235 to fit in an apartment, but that would throw away all the other parts required for the 400x400 upgrade.  The next idea is just finding a longer track, longer belt, & longer hot plate.  Ideally, the hot plate would be divided so the entire thing didn't have to be heated for every print.