Take a clip of a firefight game on Corvette, make sure that at least one Spirit dropship goes fully through.
Go into Theater and fast forward until the Spirit is leaving, then play at normal speed until the "prongs" are almost out of the shield door. Make sure there is still enough room for your camera to fit.
Play the film and position your camera between the prongs so that the ship pushes you out of the map.
There is another invisible wall just outside the ship, preventing you from going to far. Find it and position yourself so the Spirit pushes you through that as well (you may have to rewind several times to get it right)
You are now out of Corvette. You can go as far out as you want, and there is a full Covenant Corvette outside, which you can explore fully.
Once fully out, don't get too close to the ship, or you may get stuck behind an invisible wall without a Spirit to push you out (such as under the ship). You should be safe as long as you don't get too close.
It is possible to do this in Firefight mode, do the same trick by waiting for the Spirit to leave and jetpack in front of it (unlimited jetpack and invincibility help). be careful, since there is a soft kill barrier outside the ship, also if you manage to get fully out, you will die if you fall to far, which is why unlimited jetpack is required.