Hi, Hopefully you can feel the bottom edge of the hole when feeling inside the airbox, this will give positive indication that the rubeer is locating correctly in the airbox, when being fed onto the airbox hole. If it is still a struggle the other alternative, but time consuming method is to remove one carb to give more space to work and fit rubbers to airbox first with recesses greased, attached one rubber to the still fitted carb, re-attach other carb and fit the remaining rubber. I have modified an allen key, Hex key or wrench I think they call them in the US, to fit socket bolts that hold the carb manifolds to the head, as the innermost ones are pig to do with a standard length allen key - Just shorten it on the long section and the short section and it makes doing carb removal a whole lot easier.