It is actually challenging to find at the appropriate amp capacity... but I would like to put the Arduino to sleep for most of the time, so i can't be holding the transistor gate high. I want it to fail closed.
Mike's suggestion was to use a pullup to hold the gate high by default. You can use a transistor driven from the Arduino to pull the gate low, that way when the Arduino is powered down or the output pin is low, the gate is pulled high and the mosfet is on. See attached diagram. Btw you've not specified the total current drawn by the motors, so I can't say whether a single IRF540 is up to the job.