Portable will be pretty bulky / heavy... even one battery.
If it were me I'd do two batteries and store one in the trunk if there is not room under the hood.

They make other batteries that are 1/2 the size of the optima and you can fit two of them or more under the hood where you can fit them

The two battery idea doubles your storage capacity so for the money it's the best bet. Add a solar charger and you should set.

-Todd