Hmmm...tough call, if you can't just use a dirt pile. I'd try to convice the wife that you can build something thats not too ugly (use plantings, logs, landscape timbers, railroad ties (if you can get em), then plant stuff around it
On my property I have about a 50 ft. elevation change from the bottom to the the top where the house sits (10 acres). So I've put out some old appliances (the garbage guys won't take 'em), with some 2 foot diameter logs behind them, and behind them the hillside (about halfway up the property). Believe it or not, I'm actually shooting directly towards my house/barn. But, that way the neighbors can't complain, as my own property would be the first stuff hit! Any shots that might go over the "range" would likely get stopped farther up the hill, or by the heavy trees farther up, or worst case, by my house/barn!
I am (if I can afford it) gonna get a bulldozer in there and make if better, but it's really safe enough, as is.