Media/Data racks, they are all standardized.. a standard server rack is 48U high, ie it could hold 48 1U servers that look like this:
If you need to protect all your equipment, a secure mini-size rack cabinet is what you need..
http://www.monoprice.com/category?c_id=105&cp_id=10516
you can often find used cabinets/racks on craigslist or recyclers for a fraction of what they cost new, since nobody wants to ship em.
bolt all your computers, switches, equipment inside a rack and then bolt it to a floor and then put a lock on it and while it wont be completely impossible to disable or steal.. it will be alot fucking harder take alot more time than just grabbing a mini-pc off a shelf and tossing it in a bag.. espically if you torque the fuck out of everything so it needs power
tools to remove.. you can even get security bolts that stop em without the right tools.. come in with a welder and tack the bolts securing the cabinet to the concrete floor in, then they need a grinder.
sounds like you need a pretty small locking cabinet, just the NVR, PoE Switch, and perhaps cable modem/wifi, put it some place discrete, hidden inside a closet or disguised under a fake facade.. its not a big facility so if they have enough time int here they will find it unless you get very clever.. and if you get that clever, dont share it with anyone, even us