If you're like me and are a hoarder stuck working with a standard size stash in the very early game, this trick is for you. Cheap tactical vests like the Wartech MK3 and BlackRock are 3×4 slots on the outside, but 5×4 on the inside. For around 35,000-50,000 RUB a pop (or any you loot) you can get 8 free slots in your stash, long before upgrades or cases. The only downside is that this only works for smaller items, but that's not too big an issue. A barter and crafting items collection, or all your ammo and mags for example, can go into a holding rig for a free boost to space. I use 3 or 4 of them and it comes out to an extra few lines at the bottom of the stash, at basically no cost.
I assume this works with higher level rigs too, at increased rarity and price, but I haven't found any others yet. Anyway, if you're pretty tight on space I hope you find this as useful as I did.