player.inventory.containerMain.ServerInitialize(null, 12);​

This works, but is very buggy when trying to drag items around in the inventory. Is there a better way to accomplish this?

player.inventory.containerMain.capacity = 12;

Then you can send network update to player or mark container as dirty

RyanFromRust

Might be some code in here: https://umod.org/plugins/inv-lock

Well it locks whole container, in mean while he just needs half. Since we can't block specific slots, only way we can do is decrease capacity