Not sure what this is about but if you look at the inside of the prop walls it has a ribbed look, almost like it is some kind of insulation.  Perhaps to muffle the sound of the bikes motor?

That's probably just crash protection, for the scene being filmed. I'm guessing it won't be visible in the final shot.

Oh I have absolutely no doubt that it won't be visible, just wondering what was going on with it

If you look for the pictures of Jenna and Matt coming out of the box, those "buffers" are not in there and the back of the box is reinstated.  So yeah, they are probably just used for crash protection and probably help the box stay rigid while the bike goes through it.
Bill "the Doctor" Rudloff

Never mind the protection, what about that hideous monster on the left - Night of the Living Statues! Invasion from the planet Marcel Marceau! Eek!

I realize that it has been a while since this post was created, but wanted to say something anyway.

I think that the "insulation" was just the decor for the interior of the TARDIS that can also be seen here in 'The Snowmen'.

As to why it was there while filming, no idea.

Feel free to correct me or inform me that in that particular filming that it had a purpose.

They replicated the tubing from the interior for the Exterior when filming off the Interior set. This was add more detail to the bigger on the inside effect they made better in S7B!

- Tom