Your dustcap will glue to the outer edge of that piece. Once it is glued on, an outer bead should be ran around the outside of the cap. Take a look at BushJ311's or 528hz's speaker on an above post for a reference.
You're not going to be able to run an inverted cap on an MT. Well, you can if you want, but it will be coming off shortly after you do. There is far too much pressure capped off by the dustcap. You need the 2 piece, or one VERY thick cap to have enough surface area to glue to.... at least in my experience.
Basically, I sent it that way for a good reason. Your kit is designed to work with the parts supplied, not just some other cap.
If ya wanna use something else.... you'll have to rip the inner piece off, cut the former (and maybe the leadwires) shorter, then try to fit the inverted cap. I highly recommend NOT doing this.