I modified the Gridfinity Base Box to accommodate the .223/.556 ammo holder inserts that I have tagged here. The 4x4 base (your best overall option but takes more filament) works with the 4x4 insert from this contributor but you will need to scale down the height of their project by 10mm to more easily grab the ammo: [https://makerworld.com/en/models/882867-223-remington-gridfinity-ammo-storage-boxes-4x4#profileId-837365](https://makerworld.com/en/models/882867-223-remington-gridfinity-ammo-storage-boxes-4x4#profileId-837365) and the 3x4 base works with the 3x4 insert from this contributor on printables: [https://www.printables.com/model/1031478-gridfinity-223-ammo-holder](https://www.printables.com/model/1031478-gridfinity-223-ammo-holder) – What I ended up doing was taking the original model base, cutting out the mid part of it that had no additional attachments for latches etc.. I increased that slice by 20mm and put it back together. This way, when it printed, it was able to allow the lid to close with a fully loaded tray of ammunition. You can turn it upside down and you will hear the ammo move some but they will not spill out. Since this is a Gridfinity box, you can opt to take the insert out and use the box for anything else.