I would think you would want to look at 6 or 7 gauge steel for this. From a sales perspective- if the material is too thin and dents or folds over time- people are not going to ultimately be too happy with such a purchase- anything made out of "steel" tends to register with people as being "solid". Sheet metal target traps can be had for $30.00, to charge more than that for a similar-quality product would not be so easy, and the after-the-fact number of complaints goes through the roof.

Don't know what the difference in cost would be between a 9-gauge sheet and a 6-gauge sheet or the increase in cutting time, but it might be something to consider.