You can just staff it with terrible people and not build any extra platforms and etc. People can still invade though. If they're successful I think you can lose some people not on the FOB staff.
Loading your FOBs with high level staff makes those people more easily available to steal but it also makes invading FOBs extremely hard. The theft of unprocessed materials is irrelevant.
Aren't all staff who aren't under direct contract (which is usually limited about 10-15% of your total staff count) vulnerable to being stolen if an enemy reaches their target destination? That's a large proportion of your staff.