Just to clarify, are we talking about the old OTV (IBA), IOTV, Marine MTV, or...?
I made inserts for my IBA (SDS) from closed cell foam (aka. foam bed roll) which is almost the same thickness and rigidity as the kevlar soft armor inserts. I used newspaper and a sharpie to draw a rough shape of the inserts and after cutting them out and making sure they fit properly, I used them as a guide to cut the foam.
With no armor inserted the carrier will be very loose, yes.
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