You need the multitool, unless you happen to have microblocks or something non-solid to make a doorway in the force field.
You can also hook up pressure plates to send redstone signals to the MFFS projector to detect when you're about to leave or enter your base.
The above options aren't safe at all on a multiplayer server. Use at your own risk. On singleplayer they also pose a threat, although the mobs' path finding will be screwed up and it's unlikely that they'll find the pressure plate or the microblock doorway.
