They tend to be big and ugly because you needs a lot of switch to carry and, when needed, break the current of something like a motorcycle.
Many woud argue that, unlike a brushed motor, if you have a brushless system you don't need one, as long ss you have a keyswitch and brake cutouts in case the throttle becomes faulty. The controller itself should never fail in a manner that provides drive to the motor (i.e. Mosfet failure).
AFAIK though, commercial EVs use a contactor controlled by the keyswitch. Still big and ugly, but can be hidden away.
For a DIY solution you could use something like a big anderson powerpole connector with a handle. Place it where you can reach it and yank it to disconnect the battery.
That said, I am sceptical that on a runaway high powered motorcycle anyone would have the presence of mind and control to take one hand from the bars and fumble for a keyswitch or emergency disconnect. Brake cutouts would probably be most effective as you will naturally apply the brakes as a reflex.