your cavity isnt large enough.. you should be comfortable with 4inch, depending on what your putting in there.
Get something bigger, and wider.. or enlarge what you have now with suran wrap and condoms.. I dont know what your sticking in there?, Is this a store bought toy, or something you made?.. of you was having sex with a real penis ,. chances are only 4 to 6 is going to make it from that anyways... like I said, take suran wrap and making it larger.. place a condom or 2 over it so its secure, and get back there..