Some PCs have internal webbing loops which are used to attach a groin protector, which is almost always of soft armor. As you know, soft armor is capable of stopping most frag, 9mm bullets, and a few other types of bullets. Not capable of stopping rifle rounds unless they are almost entirely out of energy.
Some mfrs offer war belts which have optional soft armor inserts. I just added soft armor to my Blue Force Gear belt, and was surprised at how little the IIIA soft armor inserts weighed. Care must be taken to buy the right size belt, so that the ends (and thus armor inserts) just touch or are separated a little bit.
Even if your PC has no internal webbing for attachment of a groin protector, such can usually be attached to the PALS webbing on the exterior of the PC. This has the possible advantage of allowing the user to raise or lower the groin protector to suit their body, as opposed to the fixed internal webbing loops.
The trick here is to obtain a soft armor insert for the groin protector. The carriers, usually in UCP or possibly in Woodland or 3-color desert, are not hard to get. Hereabouts, I see the UCP groin protectors and inserts fairly often, and the price is pretty cheap compared to what I see elsewhere.
I am not sure, but some info I have read leads me to believe that the Mil offered groin protectors in two sizes. I have only seen armor inserts and/or carriers for same in a single size, so this might be BS info, or else the smaller groin protector is extremely uncommon.
Noted that some posters in this thread seem to dismiss belly wounds somewhat. Such might be understandable in a current Mil situation where immediate, skilled care was available, and also quick transport to a first-rate med facility. This level of care might not always be available, so I think that is worth considering.