Don't jump over the fence, walk around it. You can actually safely enter the yard from the street if you approach the gate from the south. The guys at the barricade shouldn't see you, neither will the guys in the building next to it, your only concern is gonna be the lone guy patrolling the building that contains the stuff you want to steal but since he keeps walking clock-wise you can easily avoid him by walking the same way.

I'm attaching an image, cyan representing my path, red all the enemies and green the box. I may be wrong about the number of sleeping dudes but it works, at least at night. The dotted line roughly represents when I was fully crouching (Ctrl + Shift) but you could probably crouch less on the yard and a little more on the street.

Did this one yesterday. If you want to jump over the fence you can still do it without raising alarm. First go to your keyboard layout in the options screen. There is an "always crouch" option available which you should set to Ctrl. It's easier on the finger when you don't have to hold the key down.

You can still jump fairly high while crouching at the same time - enough to clear the fence. Your sound gauge won't spike and it shouldn't alert any of the guards.

The only thing I had trouble with was jumping to the crawl space under roof which provides a faster entry. Just go around the front of the building, while timing your movements with the patrolling guard. He'll walk the same path and he's quite easy to avoid. Sneak into the sleeping quarters past the snoring guards, get the briefcase, then leave.

Extraction is simple; climb the watchtower at the front and go over the wall. Look for two red barrels below.

Side note: what made this mission difficult for me was misleading information. The trader mentioned the base simpler to infiltrate during the day when there were only 'two' guards, rather than at night when security is beefed-up. Of course, night is by far the best method where half the base is asleep.

Of course you can do what I do, and just kill everyone in the base. You get your box and lots of loot. I may actually have to try sneaking in sometime.