The law is the same as for real guns; transport them in a locked container. If your vehicle has a suitable trunk, put the container in there.
If the container is in view due to lack of trunk space, you can cover it with a blanket or something. I would strongly suggest the back seat at least, or between the seats on the floor. Not next to you in the passenger seat.
You must be able to show that you took reasonable efforts for it to be in a locked container and out of immediate reach if you are stopped.
You can make this even less conspicuous by using a locked container that does not look like the usual 'gun' case. A tool case comes to mind.
You never, ever, just put a gun on your seat and cover it with a blanket.
Mods, feel free to delete this if it clashes with the thread.