What are you using the motor for? is it something you could just switch off and let slow down for a direction change?. if so a third on off switch before the two relays may help. or a on,off,on switch to give the motor time to fully stop. if not then you can get a 10a hbridge from aliexpress for about a tenner.

 

planning on using a DPDT momentary switch

Using a DPDT momentary center OFF position switch and a braking resistor across the motor with suppression diodes.

 

The relay method allows nearly instantaneous stops. When the push-button is released, the DC motor acts as a generator into a short circuit and it stops quickly.

 

Using a DPDT momentary center OFF position switch and a braking resistor across the motor with suppression diodes.
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Thank you!
Why wouldn’t just the resistor work?
Why are the suppression diodes needed?
And finally, how can I size the resistor and diodes?

 

The resistor may be all you need. I'm not and expert on this but I would start at say a 20 ohm 20 watt resistor.
The diodes are there for inductive transient suppression, but if all you have is a switch, battery and a motor they may not be needed unless other sensitive electronics are connected.
Recommended is a fast recovery diode. The only part that comes to mind is a 1N5819. Maybe someone else can suggest a more suitable component.