I know atx isn't terribly portable but what is the best case for portability that can hold an atx motherboard?
Fractal Design Core 1000, or Arc Midi R2, if you can catch it on sale. Nice airflow, good looks, and solid cable management.
Cooler Master Scout is worth a look
thermaltake commander is surpisingly small and light for atx at about $50 it is a decent case on a budget.