A box controller is a controller that is used to play Super Smash Bros. Perhaps the two main pioneers of the category are the Smash Box and B0XX.
The Leme Tech One is a Smash controller that runs on the proprietary Leme Tech One firmware, which is HayBox-based and HayBox-compatible.
Cubs made a hybrid, the Cubstraption, that has a GameCube controller on the left and half of a box controller on the right.
The Glyph is a modular all-button controller that is available in layouts for games from Smash to Street Fighter.
The Oblong is a mini rectangle controller that comes in different colors, but more interestingly, there is an extra button.
The FightBox R1 is FightBoxArcade's answer to the recent influx of box/rectangle controllers that are growing on the Smash community.
The Superslab Digital Combat Controller is a smaller LED controller by Prong for Super Smash Bros players.
The Kraken is SaveYourGames' debut in the world of Smash controllers, but SaveYourGames is already experienced in the fighting game scene.
It's nice how the Bubblebox is on the smaller side of things, but it still has ample width in a way that sets it apart from competitors.