We can already remove the medkit and the stim patches. They don't have an R or F rating, so just pack it in your regular bag. That's already two removed.
I've also removed the firearms repair kit. That's also not an illegal item and can also be transported in the regular bag (Clem, me or Sam can take it if it gives problems for O'Connor to have a firearms repair kit while he's not allowed to use firearms).
And it is partly a security check problem. I thought the box was to handle the illegal items we need at the other side, so if we can take our regular items (anything with rating R or lower we have licenses for) in our personal bags (check-in luggage, not hand luggage of course), we only need the box for the F-rated items (and the R-rated items from O'Connor due to his criminal SIN).
ammo boxes for pistol ammo aren't all that big (
http://westernfirearms.com.au/storage-ammo-cases-pistol-ammo-box-50-round-40-10mm-45-acp-p50-45-p-3270.html is one to put .45 ammo in, some of the biggest there is in pistols). It doesn't look all that big for 50 rounds. If we assume that pistol and machine pistol ammo has a similar size (the only difference between the two is the rate of fire, the bullets of a 9mm pistol and a 9mm machine pistol are the same if I recall correctly), we have the following:
1 light, 2 heavy pistols
1 machine pistol
2 assault rifles
2 katanas
2 knifes
1 stun baton
5 flash-bang granades
Tag Eraser x1
Firearm Repair Kit x1
Medkit R6 x1
Stim Patch R3 x2
Zen x6**
380 rounds pistol ammo from O'Connor
120 rounds pistol ammo from Sammy
50 rounds pistol ammo from Clem
200 rounds rifle ammo (O'Connor)
100 rounds rifle ammo (clem)
That's quite a lot of ammo.

Now I've just wrote down all I had, but I can probably remove quite a lot and keep 45 rounds in total (already loaded in 3 clips, so no boxes needed), as I don't need all that much (3 clips from a pistol don't take that much space). And if I can take my katana and my pistol in my checked-in luggage, I don't need any space in the special box at all, except for the extra ammo (I assume firearms have to be transported without ammo). Also if it's really needed, I can buy regular ammo in LA, as I'm quite sure they have some there, limiting what I need to 30 rounds of SnS (2 loaded clips). I can also give some of that to Clem if needed, as we both have a heavy pistol.
So if I and Clem can use our own bags to transport our R-rated items instead of this box we keep:
1 light pistol
1 machine pistol
2 assault rifles
1 knife
5 flash-bang granades
Tag Eraser x1
Zen x6**
380 rounds pistol ammo from O'Connor
50 rounds pistol ammo from Sammy
50 rounds pistol ammo from Clem
200 rounds rifle ammo (O'Connor)
100 rounds rifle ammo (clem)
I have kept the assault riffle from Clem in the box, even though it's R rated, as assault rifles are frowned upon. This should fit in a 1x1x2m box if we pack it in tight and use optimalisation (like put all the ammo together packed as tightly as possible, instead of by person).