The only way I have come across is equipment with pockets. They add one more slot. Have found the pocket feature on AVI utility chips, and on body armor.

Good luck,

Equipment with pockets: if I remember correctly, up to 2 slots per equipment piece. Might need Whizkid 3 for crafting that.

Utility chips may add 1 more.

Just throw these goldern small medkits and stims away if you come across big medkits and military stims. And don't hold onto mod packs too much, use these.

And everything will gonna be OK. . .

"Pockets" is a different bonus, it gives you 50% faster consumables usage (medkits, grenades, etc.).

The bonus on armor you're looking for is "Compartments" which gives 2 inventory slots, it can spawn naturally, you can reroll AV1/2/3 body armor to get it, or you can try crafting a body armor from the crafting station.
Modding any body armor with Power mod pack also offers to pick "Carrier" bonus, which adds 1 inventory slot.

Finally, the Refueling Base special level always has a Backpack item, which is a permanent +2 inventory slots upgrade.

Using a P-Mod on your body armour can give it the "Carrier" trait, which adds 1 slot, no Whizkid needed. I use this one pretty often as a marine.

Both The Pit and Refuelling Base have the Backpack, you will get one or the other from Europa 5. I find Refuelling Base **much** harder than any other special area in Europa, but I'll usually go to The Pit as a Marine if it comes up.

You don't need to carry all that much ammo. I usually keep around 150 9mm (or w/e) and about 50 shells and 50 .44. You can probably carry less auto ammo if you wanted since it's abundant.