Get the smallest bag from Handshake and use them to store your guns. That's what I did when some dude dropped hundreds of MK18 when I was new in the game. Got lucky and managed to get 12. Saves ton of space. You can also put small valuable items in the bag. Now that all my traders are Tier 3, I gave them all away since I can just buy it if I lose it.
And also, there's absolutely no need to store food and drinks to your locker since you can always get them on dead AIs. Medical items can be put in EMT bags to save more space.
Remember the dupe glitch thing? That money came from that. People are running around with hundreds of millions and now they are tossing it away because the game is getting wiped soon before the big update, so basically these money are sort of useless now.