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DISCLAIMER: I take no credit for this, as I'm sure I'm not the first person to find it on shi no numa by any means. I'm simply posting this for anyone who hasn't figured it out yet.
There's only 2 room you can stack up more than 2 weapons in, the fishing hut and IIRC, the storage room, but I could be mistaken. You need to make sure the box is right next to the perk-a-cola machine.
1) Open the box
2) Run to perk machine, buy perk when the box settles on a weapon
3) Sprint back to box while drinking the bottle and press x to swap.
4) Enjoy the extra weapons
Again, I take no credit for this. Just passing along info.