I wouldn't get anything that attaches to the plate carrier, cuz then you're stuck with it 100% of the time (unless you ditch your vest).
Based on the description of the situation you experienced it just sounds like you need a 2nd, small format assault pack. Leave the non-game stuff in one, game the other.
You can find good deals on LBT and Eagle packs if you dig around; buying a cheap pack defeats the purpose if its not comfortable and limits your ability to carry stuff/use it.
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