For my cap and ball sixguns I use a large box used to ship computer paper, the kind with the lid that slips over the lower portion.
I cut a piece of 3/4 inch plywood, cut to fit just inside the box. This I place at the very back of the box.
Then I add three inches of old newspaper, flat not rolled or crinkled.
Then another piece of 3/4 inch plywood. Then another layer of paper, and so on --- alternating layers of plywood and newspaper.
When the paper's about two inches above the edge of the box, I place a final piece of 3/4 inch plywood on top of that and stand on it, compressing it down to flush with the edge of the box.
You may have to add or subtract a bit of paper, to get flush with the edge under compression.
When you can get the topmost piece of plywood flush with the top of the box, jump off it and huriedly put the lid on.
Now, get someone else to stand on it, then duct-tape the lid all the way around. Have them got off the box now, and put four or five lengths across the front of the box, long enough so both ends reach around the back of the box.
That combination of newspaper and plywood layers will stopa lot of balls.
Yes, I know that your .60-caliber rifle is more powerful than my cap and ball revolvers. However, even the speediest revolver ball or conical will be stopped within the first five inches or so. Most computer paper boxes are 12 inches deep, so your .60 ball should be stopped well before the ball reaches that last sheet of plywood.
Such a box lasts me about a year of cap and ball sixgun shooting, less with your behemoth.
I just pin or tape a target to the front of the box.
Repairs to the box are made with duct tape, until it reaches such disrepair that duct tape will barely keep it held together.
It's easy to remove the lead as there's no dirt involved --- just newspaper scraps, confetti and woodchips in the center. Wear gloves to avoid splinters.
I sort through the guts at the local gravel pit. Do an autopsy on it in the house and you'll find paper and wood shreds all over the house for weeks!
On the last box I built, I hedged my bet a bit. I found a piece of heavy gauge steel on the local scrap pile, which I placed at the very back of the box. It looked like the steel cover for a breaker box or something.
Anyway, I don't expect many lead bullets will get through layers of newspaper and 3/4" plywood, and THEN pierce that heavy steel cover.
You don't have to use plywood of course. In your case, you may find that lengths of 2X4 or 2X8s, side by side in the box, work much better.
This system gives you a portable box. Mine weighs about 35 pounds --- heavy enough to resist toppling in all but the strongest winds, yet light enough to carry out to the range.
Lately I've been thinking of getting fancy, and mounting the box to one of those folding luggage carriers that people use at the airport: pull the bullet trap out of the back of the pickup, swing up the handle, lock it into place, and WHEEL the box to the proper distance! Neat, huh?
Haven't done that yet but I expect to try it soon.
In short, I should think that layers of 2-inch thick wood and newspaper would stop the ball for recovery later.
Best of all, except for the duct tape the whole bullet trap is made of materials typically discarded.
Let me know if the box stops your .60 balls. Mine has stopped an occasional lead .45 Auto roundnosed bullet of 230 grains, at about 850 fps, at 25 yards.
However, I'm sure .357 or .44 Magnum, or most any jacketed rifle bullet, would zip through it. It's not a catch-all.
I made it to capture pure lead revolver balls for recycling. Works great.