That 25.95% assumes you've carefully packed each layer, arranging them perfectly for maximum efficiency. For "random" packing, that number is closer to 35%.
So, closer to 7,000 balls, practically speaking.
Yeah, I didn't really feel like trying to get too into it so I figured a cube but went slightly smaller on all sides to account for balls nestling into the crooks between adjacent balls.
I mean, that's how most of my "calculations" go anyway. Estimate, estimate, estimate! (Then round up to the safe side) :)
I swear there's this one group of bushes on hole #7 at my normal Saturday morning group course (not my home course, strangely enough) that someone in my group will always find themselves hitting into. It's about 20 yards short right of the green on a longer par 5 so they're usually in there trying to reach in 2. But I swear every weekend someone is in there and every weekend while we search for that ball we find between 5-10 balls that AREN'T the playing partner's ball! Hilarious. And it's just a small group of bushes. Maybe 150 sqft spot. It's surrounded by rough otherwise which is easy to get out of.