He sealed it in himself, IMO!
The seal formula was placed on HIS BELLY so...
And secondly, he said, after the sealing of the first part of kyuubi, that the chakra is so powerful, its making him feel numb.
So was the seal on hiruzens belly from orochimaru's arms, so were the seals on hiruzens clones from hashi and tobirama.. the user and the victim will get sealed mingled with each other hating each other into eternity..
And besides if he would've sealed it in himself (which is not himself, but his body) that half would've already returned to kyuubi cause his body is dead..
If a jinchuuriki dies the bijuu dies with him but the bijuu will revive..
Minato talked about sealing the kyuubi completely IN THE REAPER GOD thingy so he wouldn't return, but that would be impossible for him to do with the power the kyuubi had and it would disturb the bijuu balance in the world or something..
And i'm 1000% certain next time we see naruto he will get that half back.. also because he else he could never compare with the likes of madara and hashirama..