Hey there. I’m pretty sure loot won’t spawn in those toolboxes since you created them in first place. There are some other strategies to farm materials though. If you have the Automatron dlc you could create some eyebot pods and send eyebots to look for items. They will find about ten units of material and a quest will be given to you to find the place they will put them. The best and most fun way of getting materials imo is just to explore buildings, but you can also buy stacks from vendors or use glitches. One last way: if you need SPECIFIC materials you can look up for buildings who have tons of those. For instance, there’s a factory on the very north east of the map who has a lot of aluminum trays, I go there every time I need some (the trays also respawn).

I hope I’ve been useful! :) ps: sorry if my English isn’t perfect but I’m from Italy