I remember going through this process as well when I was creating my first characters and NPCs. I was trying to figure out how to give someone a higher Security score. I went through the whole book and there’s just nothing on it. Some NPCs just have a NPC ability that gives them higher security… but nothing that players can access. The only option for players is BTS on their armor.
Also, don’t forget about the Fairy Fire hacking program. A hacker can spend a bunch of momentum on this program to give everyone on their team (up to LONG range!) +5N Interference Soak. That’s quite a lot of soak potential.
There’s a lot of ways you could house-rule this, and there have been some great suggestions. Just to pile-on my own suggestion, I would say that allowing players to modify their armor by increasing its BTS value would be a good method. Have the armor modification have a cost and a Tech roll associated with successfully installing it.