I'm a big fan of your project and you did an excellent job so far! Here is an image I did by adding a mirrored lower part of the ammo belt to creat a whole:
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It seems that you need to lengthen the sewing with 12mm, but all the other parts looks good to me.
I think the double layered ammo belt (around 2-3mm total thickness) without cardboard is the best choice because when you put the belt on you it should be tight and not loose, this way the ammo pouches won't hang on the belt. But if you use thinner sinthetic material, then the cardboard support is an excellent addition.