The ventriloquist has his hands full oops I mean his mouth full when it comes to the distant voice. Not only does the ventriloquist have to learn to speak for his friend in his normal vent voice, but he must also be able to speak in a near-distant, distant, and muffled voice. Now the near-distant voice would give the impression that someone is nearby but not immediately along side of him, they could be on the other side of the room. As for the distant voice, someone could be outside of the window, next door, upstairs in the attic or downstairs in the basement. The muffled voice gives the impression of someone who is behind closed doors, your character can still be in a trunk or there could be a blanket over him, or, as the Great Lester used in his vaudeville act 'on the other end of a phone line'. These are all variations of the same technique. When you master the 3 techniques you will find that there are innumerable types of distant voices as you can make the voice come nearer or further away.
How does a ventriloquist make the voice seem distant? It is by proper use of his diaphragm and compressing of his voice muscles when speaking so the vibration of the vocal chords are prevented from the voice actually sounding like its coming out of the oral or nasal cavity. Picture yourself lifting one end of a couch and when you are doing it make the sound of "aaaahh", like when you are lifting something very heavy, this is the sound that you are looking for. In your practice, actually lift something that is almost impossible to lift by yourself. Now concentrate on the sound that you make trying to do that. There is your sound. OK. Now depending on how hard you squeeze gives the illusion of being further and further away.
I would not recommend practicing too much at the beginning due to the strain that one puts upon their throat muscles. As you progress with this sound, your muscles will get stronger and stronger and your practice time will get longer and longer.