Transportation of firearms in a vehicle is covered under
MGL C140 S131c, and makes no mention of ammunition.
The locked storage of ammo is from the fire regulations in
CMR 527 S13.
13.04 (e) is the requirement for locking exempt quantities of ammunition if stored in a building or structure - not a vehicle.
13.06 is the transportation section, and in (b) states that 13.06 doesn't apply to small arms ammunition or components.
In my not a lawyer opinion, this means you don't have to lock up ammo in your car. However, if in the interests of avoiding ever being declared an unsuitable person on your next renewal (not that this applies to the OP) because you got pulled over for speeding with a pile of ammunition loose in the back seat, you decide you would like to lock it up, than I would say that a toolbox would do nicely!
Depends on your level of risk tolerance, I suppose.