There is a lot of danger in a DIY bullet deflector. This is especially true with rifle traps.

I would go with a barrier made from railroad cross ties. Stacked up about five feet high, 2-3 deep, in a U-shape works well. Depending on how much shooting is done there, you may have to replace one or two of the ties every couple of years. I went six feet high at my farm and then pushed dirt behind it in the unlikely event a round would penetrate two layers of ties. Bought the ties for less than $200 used and had a friend help me place them.