Physically, a removable R2 would be almost impossible. If I recall from the cross-sections book, the only way the astromech droid "fits" is that his torso cylinder and legs stay down in the more bulky part of the hull, while his dome telescopes up out of the torso cylinder and up through the hole at the top of the starfighter.
It's one of the aspects of the N1 design that I always thought was pretty ridiculous.