While you're here, you might as well liberate the outpost. There's a wooded slope to the south, and an outbuilding to the east- both are good for picking off wayward patrollers.
Once you've thinned the perimeter, this is a surprisingly easy compound to stealth. There are shrubs and cover everywhere, and the enemies are too spread out to mob effectively. Boomer can be a handy companion here, to tag enemies you might have missed across the sprawling compound.
One strategy to always keep close to the main building. This will keep the structure between you and many of the hostiles, and you can always duck back into cover at a moment's notice. Clear all enemies (roughly a baker's dozen) to proceed.
Head to the hangar. There's a map of the Seed Ranch area outside, on some black trunks.
Enter the hangar- inside there's a fenced of corner toward the south.
Use the crates on teh near side to jump over the chainlink. You'll find the safe on the other side.