The MTV used MTV cut panels, which are two pieces (front and back) not including accessories. The IBA (interceptor OTV) uses a front opening configuration that is a 3 piece design. Thus a set of IBA inserts will not work with an MTV or IMTV carrier as the panels are a different layout (not to mention the differences in the cut itself).
OP, the IBA is a specific cut of armor separate and apart from BALCS/SPEAR, IOTV, MTV, or the various concealable cuts. You need something specifically designed to work with IBA/OTV cut armor. There are several options though they are getting harder to find these days as the system has been phased out by the military in favour of the IOTV and more often than not plate carriers. However they are out there as the system is still used by LE and plenty of foreign .mils and many are looking to upgrade what they have. The TAG Spartan, the BEEZ OTV carrier (several configs), the DBT Predator OTV (with the one piece front conversion bags), etc are all options and they convert the system from an older front open to a more modern side opening system while also reposition the panels to make up for the deficiencies of the cut (which is really the systems only big problem as the armor itself is still great and up to date so to speak).