...or Make It Simpler, Stupid.
Forget the mangle, it doesn't need to be that complicated.
All that's really needed is a rectangular tube the size of a bottle, to store the squashed bottles in, closed at one end, with a crushing-flap and a sliding door at the other end. This'll be easier to explain with a picture:
(I used Google Sketchup to create this drawing - one of my favourite tools, and free!)
To use the crushed-bottle store, you pull up on the handle to open the hinged flap, place the plastic milk bottle onto the green panel, then close the flap gently on top of the bottle to hold it in place. Next pull the green slide panel out. If there aren't many bottles inside so far, the bottle will just drop in. Once the level of bottles reaches the green panel, new bottles will rest on top of the stack. You can now press down on the flap handle, crushing the new bottle (and re-crushing the ones underneath). While holding the flap pressed down, push the green sliding panel back in to trap the crushed bottles inside.
Emptying it could be a bit tricky. Open the flap and slide out the panel, and all the compressed bottles will shoot out. It might be fun! (once or twice).
If the entry hole in the bottle-bank were wide enough to take a bottle sideways, it might be possible to take the crushed-bottle store to it, open the flap, hold the end up to the hole and then release the green sliding panel, but the bottle bank hole is only wide enough to put bottles in long-ways. Hmm...