As a sec main with around 300 hours in sec, I think I can provide some pointers on what I like in maps (especially lowpop since that's one of my favorite map types).
Regarding which maps I think you should look at when designing your map's sec department, I would say that Packed, Saltern, and Omega are the best ones for a compact yet functional design. I specifically like Packed's Armory and Detective's office, Saltern's HoS office and Main Area, and really just everything about Omega's sec design minus the maintenance Detective's office. I also really like Meta's HoS office.
For minimum equipment, I recommend having enough gear to supply 2-3 officers, so 2 lockers and 3 suit lockers in the armory, alongside only needing 2 ballistic safes in the armory (a drozd and lecter safe). However, I honestly don't recommend leaving any hand-operable items lying around, they often just clutter up the area and make it harder to find things. The only things that should be easily accessible from the main area should be 2 electronic barriers. As for the armory, I recommend 1 sec techfab, 1 lecter safe, 1 drozd safe, 3 suit lockers, a flasher, an implant crate, and 1 rack with 2 bulletproof vests and 2 riot suits. Please don't use windoors and directional windows, those just feel really clunky to use.
Also, please put the bomb suit locker outside of the armory and in the locker room, being able to respond to a bomb on short notice is really important.
As for conveniences, I really like when sec has a crew monitoring console so that its easier to track people (especially low-level fugitives), a toolbox in the warden's office with all the tools, and a medsec area for doing basic medical treatment of convicts that get hurt. As well, I also like when sec has access to a roundstart advanced medkit, but if there's already going to be a medsec area it doesn't really have to be added.
As for the most important element of security, well, sec is oriented around simply being robust and having the right gear for the job when needed. Things like having the right weapons, armor, or gear can make all the difference.
This also goes into the thing about where I spend my time, because I usually am out patrolling, and only ever stop by sec when I feel like I need a break for a bit or if nothing is happening to have a nice place to RP and chat a bit.
As for what I haven't seen before that I would like to see, I think that maybe placing a .35 ammo box in the locker room somewhere so that officers can more easily refill their magazines without having to bug the warden or HoS would be cool, though it might end up with someone taking it and that would probably be pretty bad.
