Up in the Loft??? Guess any use of Sand and Cement will need to consider weight too, as that could get heavy for loading up a small footprint.
Splash Back could be ugly, though small caliber's (under 380) might be the limit against a wood front, even with a Rubber cover.
Definately, an angled "Bullet Trap" would be best, and probably the lightest. If you did this, even upstairs in the loft, you could have a Steel plate on the floor, and 2 angled plates in front, forming a ">" trap, similar to how the Indoor Ranges are constructed. And if you shoot lead, you could "Recycle" and remold your bullets
I would not consider Fiberglass Insulation, it's just not that dense to "trap" bullets.
Dirt, Sand, or some other "Media" would be better, and the suggestion to have a space behind to check for signs of damage/failure is a good one too.
I have given serious though to a "Home Range" when I move, and get some property...
For me, I am thinking underground, off from the Basement, maybe using those big Cement Pipes for the tunnel.
One thing to remember is to ensure good ventilation, blowing from behind, downrange, to keep lead/powder residues away from the shooter. Also, a well vented area will help with the increased spikes in Sound Pressure level's when shooting.
Enjoy!